media: pixfmt-v4l2-mplane.rst: fix types

The v4l2_pix_format_mplane documentation still had 'enum's as types.
Replace by __u8 and add a reference to the enum.

Also put ycbcr_enc and hsv_enc in a union.

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