[media] DocBook: V4L: add V4L2_SDR_FMT_CS8 - 'CS08'

V4L2_SDR_FMT_CS8 is complex signed 8-bit sample format, used for
software defined radio devices.

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