media: v4l2-common: add pixel encoding support

It is often useful to figure out if a pixel_format is either YUV or RGB
especially for driver who can perform the pixel encoding conversion.

Instead of having each driver implement its own "is_this_yuv/rgb"
function based on a restricted set of pixel value, it is better to do
this in centralized manner.

We therefore add a pixel_enc member to the v4l2_format_info structure to
quickly identify the related pixel encoding.
And add helper functions to check pixel encoding.

Signed-off-by: Benoit Parrot <bparrot@ti.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>