[media] v4l: uapi: add SDR format RU12LE

V4L2_SDR_FMT_RU12LE - Real unsigned 12-bit little endian sample
inside 16-bit (2 byte). V4L2 FourCC: RU12.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>