media: imx-pxp: Add media controller support

Register a media device for the PXP, using the v4l2-mem2mem MC
infrastructure to populate the media graph. No media device operation is
implemented, the main use of the MC API is to allow consistent discovery
of media devices for userspace.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>