media: imx: csi: Lookup upstream endpoint with imx_media_get_pad_fwnode

Fix the 1:1 port-id:pad-index assumption for the upstream subdevice, by
searching the upstream subdevice's endpoints for one that maps to the
pad's index. This is carried out by a new reverse mapping function
imx_media_get_pad_fwnode().

Signed-off-by: Steve Longerbeam <slongerbeam@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>