[media] DocBook: V4L: add V4L2_SDR_FMT_RU12LE - 'RU12'

Document V4L2_SDR_FMT_RU12LE format. It is real unsigned 12-bit
little endian sample inside 16-bit space. Used by software defined
radio devices.

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