media: staging: rkisp1: add document for rkisp1 meta buffer format

This commit add document for rkisp1 meta buffer format

Signed-off-by: Jacob Chen <jacob2.chen@rock-chips.com>
Signed-off-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>