power: battery: Generic battery driver using IIO

power: battery: Generic battery driver using IIO

Driver to allow use of the ADC drivers supported by the IIO
subsystem for battery status monitoring. Connecting this
driver to the relevant IIO device requires registration of
the appropriate iio_map structure array by the IIO device
driver (usually from platform data).  If specified the driver
will also make use of a gpio to provide interrupt driven
notification that the battery is fully charged.

In last version:
Addressed concerns raised by lars:
a. made the adc_bat per device.
b. get the IIO channel using hardcoded channel names.
c. Minor issues related to gpio_is_valid and some code
   refactoring.

In V1:
Addressed concerns raised by Anton:
a. changed the struct name to gab(generic adc battery).
b. Added some functions to neaten the code.
c. Some minor coding guidelines changes.
d. Used the latest function introduce by lars:
   iio_read_channel_processed to streamline the code.

In V2:
Addressed concerns by lars:
a. No need of allocating memory for channels.Make it array.
b. Code restructring, coding style and following kernel guidelines changes
   suggested by him.

In V3:
Addressed conerns by Anton:
a. Added the copyright.
b. Coding guidelines changes suggested by him.
c. Added Makefile and Kconfig

Signed-off-by: anish kumar <anish198519851985@gmail.com>
Acked-by: Anton Vorontsov <cbouatmailru@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>