media: v4l2-flash-led-class: Create separate sub-devices for indicators

The V4L2 flash interface allows controlling multiple LEDs through a single
sub-devices if, and only if, these LEDs are of different types. This
approach scales badly for flash controllers that drive multiple flash LEDs
or for LED specific associations. Essentially, the original assumption of a
LED driver chip that drives a single flash LED and an indicator LED is no
longer valid.

Address the matter by registering one sub-device per LED.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com> (for greybus/light)
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>