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>