media: marvell-ccic: mmp: get rid of mmpcam_devices list

The sole reason for this was so that it would be possible to get a
mmp_camera from a platform_device in suspend/resume/remove hooks, because
drvdata used to be used by the v4l core.

Since commit 95db3a60 ("[media] v4l: Add a media_device pointer to the
v4l2_device structure") it is no longer case and thus we can make things
a great deal simpler.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>