Document V4L2_SDR_FMT_CU16LE format.
It is complex unsigned 16-bit little endian IQ sample. Used by
software defined radio devices.

Cc: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>