V4L/DVB (12149): videodev2.h: Reorganize fourcc table

With the changes this file suffered along the time, things got a little disorganized.
In particular, V4L2_PIX_FMT_YVYU were shown as a device-specific format, instead of
yet another variant of YUV.

There's no functional change on this patch. It just adds some comments and reorder
fourcc formats to their proper places.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>