media: v4l2-subdev: add v4l2_subdev_get_fwnode_pad_1_to_1

Add a convenience function that can be used as the .get_fwnode_pad
operation for subdevices that map port numbers and pad indexes 1:1.
The function verifies the endpoint is owned by the subdevice, and if
so returns the endpoint port number.

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>