gpu: ipu-v3: Do not bail out on missing optional port nodes

The port nodes are documented as optional, treat them accordingly.

Reported-by: Martin Fuzzey <mfuzzey@parkeon.com>
Reported-by: Chris Healy <Chris.Healy@zii.aero>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Fixes: 304e6be6 ("gpu: ipu-v3: Assign of_node of child platform devices to corresponding ports")