[media] DocBook media: rewrite the Colorspace chapter

The colorspace chapter in the V4L2 Specification was always poorly
written. This patch rewrites it, documenting the new Y'CbCr encoding
and quantization defines and going into much more detail with respect
to how colorspaces are used and what it all means.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>