media: leds: as3645a: Add LED flash class driver

Add a LED flash class driver for the as3654a flash controller. A V4L2 flash
driver for it already exists (drivers/media/i2c/as3645a.c), and this driver
is based on that.

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