backlight: new driver for the ADP8860 backlight parts

The ADP8860 combines a programmable backlight LED charge pump driver with
automatic phototransistor control.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>