test: dm: test-fdt: Add checks for uclass_get_device_by_endpoint()

test: dm: test-fdt: Add checks for uclass_get_device_by_endpoint()

This is a new DM core helper. There is now a graph endpoint
representation in the sandbox test DTS, so we can just use it to verify
the helper proper behavior.

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