media: dt: bindings: Document DT bindings for Analog devices as3645a

Document DT bindings for Analog Devices as3645a flash LED controller which
also supports an indicator LED.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>