dt-bindings: display: convert Arm Mali-DP to DT schema

The Arm Mali Display Processor (DP) 5xx/6xx is a series of IP that scans
out a framebuffer and hands the pixels over to a digital signal encoder.
It supports multiple layers, scaling and rotation.

Convert the existing DT binding to DT schema.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220506140533.3566431-11-andre.przywara@arm.com