gpu: ipu-v3: document valid IPUv3 compatibles and extend for i.MX6 QuadPlus

Document the valid compatible strings for the IPUv3.

On i.MX6 QuadPlus the IPU needs to know which PRG has to be
used for this IPU instance. Add a "fsl,prg" property containing
a phandle pointing to the correct PRG device.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>