media: Documentation: v4l: move table of v4l2_pix_format(_mplane) flags to pixfmt-v4l2.rst

The table of the flags of the structs
v4l2_pix_format(_mplane) is currently in pixfmt-reserved.rst
which is wrong, it should be in pixfmt-v4l2.rst

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>