dm: core: Add a helper to retrieve devices through graph endpoints

dm: core: Add a helper to retrieve devices through graph endpoints

There are already several helpers to find a udevice based on its
position in a device tree, like getting a child or a node pointed by a
phandle, but there was no support for graph endpoints, which are very
common in display pipelines.

Add a new helper, named uclass_get_device_by_endpoint() which enters the
child graph reprensentation, looks for a specific port, then follows the
remote endpoint, and finally retrieves the first parent of the given
uclass_id.

This is a very handy and straightforward way to get a bridge or a panel
handle.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>