media: device property: Add a function to test is a fwnode is a graph endpoint

Drivers may need to test if a fwnode is a graph endpoint. To avoid
hand-written solutions that wouldn't work for all fwnode types, add a
new fwnode_graph_is_endpoint() function for this purpose. We don't need
to wire it up to different backends for OF and ACPI for now, as the
implementation can simply be based on checkout the presence of a
remote-endpoint property.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>