iio: gyro: Add driver for the MPU-3050 gyroscope

iio: gyro: Add driver for the MPU-3050 gyroscope

This adds a new driver for the Invensense MPU-3050 gyroscope.
This driver is based on information from the rough input driver
in drivers/input/misc/mpu3050.c and the scratch misc driver
posted by Nathan Royer in 2011. Some years have passed but this
is finally a fully-fledged driver for this gyroscope. It was
developed and tested on the Qualcomm APQ8060 Dragonboard.

The driver supports both raw and buffered input. It also
supports the internal trigger mechanism by registering a trigger
that can fire in response to the internal sample engine of the
component. In addition to reading out the gyroscope sensor
values, the driver also supports reading the temperature from
the sensor.

The driver currently only supports I2C but the MPU-3050 can
also be used from SPI, so the I2C portions are split in their
own file and we just use regmap to access all registers, so
it will be trivial to plug in SPI support if/when someone has
a system requiring this.

To conserve power, the driver utilizes the runtime PM
framework and will put the sensor in off mode and disable the
regulators when unused, after a timeout of 10 seconds.

The fullscale can be set for the sensor to 250, 500, 1000 or
2000 deg/s. This corresponds to scale values of rougly 0.000122,
0.000275, 0.000512 or 0.001068. By writing such values (or close
to these) into "in_anglevel_scale", the corresponding fullscale
can be chosen. It will default to 2000 deg/s (~35 rad/s).

The gyro component can have DC offsets on all axes. These can be
compensated using the standard sysfs ABI property
"in_anglevel_[xyz]_calibbias". This is in positive/negative
values of the raw values, so a suitable calibration bias can be
determined by userspace by reading the "in_anglevel_[xyz]_raw"
for a few iterations while holding the sensor still, create an
average integer, and writing the negative inverse of that into
"in_anglevel_[xyz]_calibbias". After this the hardware will
automatically subtract the bias, also when using buffered
readings.

Since the MPU-3050 has an outgoing I2C port it needs to act as
an I2C mux. This means that the device is switching I2C traffic
to devices beyond it. On my system this is the only way to reach
the accelerometer. The "sensor fusion" ability of the MPU-3050
to directly talk to the device on the outgoing I2C port is
currently not used by the driver, but it has code to allow I2C
traffic to pass through so that the Linux kernel can reach the
device on the other side with a kernel driver.

Example usage with the native trigger:

$ generic_buffer -a -c10 -n mpu3050
iio device number being used is 0
iio trigger number being used is 0
No channels are enabled, enabling all channels
Enabling: in_anglvel_z_en
Enabling: in_timestamp_en
Enabling: in_anglvel_y_en
Enabling: in_temp_en
Enabling: in_anglvel_x_en
/sys/bus/iio/devices/iio:device0 mpu3050-dev0
29607.142578 -0.117493 0.074768 0.012817 180788797150
29639.285156 -0.117493 0.076904 0.013885 180888982335
29696.427734 -0.116425 0.076904 0.012817 180989178039
29742.857422 -0.117493 0.076904 0.012817 181089377742
29764.285156 -0.116425 0.077972 0.012817 181189574187
29860.714844 -0.115356 0.076904 0.012817 181289772705
29864.285156 -0.117493 0.076904 0.012817 181389971520
29910.714844 -0.115356 0.076904 0.013885 181490170483
29917.857422 -0.116425 0.076904 0.011749 181590369742
29975.000000 -0.116425 0.076904 0.012817 181690567075
Disabling: in_anglvel_z_en
Disabling: in_timestamp_en
Disabling: in_anglvel_y_en
Disabling: in_temp_en
Disabling: in_anglvel_x_en

The first column is the temperature in millidegrees, then the x,y,z
axes in succession followed by the timestamp. Also tested successfully
using the HRTimer trigger.

Cc: Nick Vaccaro <nvaccaro@google.com>
Cc: Ge Gao <ggao@invensense.com>
Cc: Anna Si <asi@invensense.com>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Crestez Dan Leonard <leonard.crestez@intel.com>
Cc: Daniel Baluta <daniel.baluta@intel.com>
Cc: Gregor Boirie <gregor.boirie@parrot.com>
Cc: Peter Rosin <peda@axentia.se>
Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>