[media] em28xx: shed some light on video input formats

CbYCrY has been identified by looking into the tvp5150 driver and the
saa7115 datasheet.
YUV formats have been verified with em2765 + ov2640 (VAD Laplace webcam).
RGB8 formats have been verified with em2710/em2820 + mt9v011 (Silvercrest
webcam 1.3mpix).
I also did some cross-checking with these two camera devices and 0x08-0x0b
are at least 16 bits per pixel formats on em2710/em2820, too, and
0x0c-0x0f are at least 8 bits per pixel formats on em2765, too.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>