iio: adc: New driver for AD7280A Lithium Ion Battery Monitoring System

The AD7280A monitoring system contains all the functions required for
general purpose monitoring and maintenance of stacked
lithium ion batteries as used in hybrid electric vehicles,
battery backup applications, etc.

Changes since V1:

Make cell channels all type IIO_IN_DIFF, update documentation accordingly.
Remove unused and redundant defines.
Use SI units where applicable.
Remove unnecessary wrapper function.
Remove redundant initialization.
Add comments where requested.
Revise event handler.
Use const where applicable.

Changes since V2:

Remove redundant adc.h include file, scheduled for removal.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>