[media] ad5820: Add driver for auto-focus coil

This adds support for AD5820 autofocus coil, found for example in
Nokia N900 smartphone.

Signed-off-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>