[media] media: i2c/adp1653: Documentation for devicetree support for adp1653

Documentation for adp1653 bindings.

Signed-off-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>