iio: frequency: New driver for AD9523 SPI Low Jitter Clock Generator

iio: frequency: New driver for AD9523 SPI Low Jitter Clock Generator

Changes since V1:

Apply Jonathan's review feedback:
Revise device status attribute names, and split documentation into two sections.
Add additional comments, and fix indention issues.
Remove pointless zero initializations.
Revise return value handling.
Simplify some code sections.
Split store_eeprom and sync handling into separate functions.
Use strtobool where applicable.
Document platform data structures using kernel-doc style.

Use dev_to_iio_dev
write_raw IIO_CHAN_INFO_FREQUENCY: Reject values <= 0
Make patch target drivers/iio

Changes since V2:

Use for_each_clear_bit() and __set_bit() where applicable.
Add descriptive comment.
Avoid temporary for struct regulator.
spi_device_id name use ad9523-1, ad9523 will be added later.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>