dt-bindings: display: convert PL110/PL111 to DT schema

The Arm PL110 and PL111 are IP blocks that provide a display engine with
an LCD interface, being able to drive a variety of LC panels.

Convert the binding over to DT schema, to the DTs can be automatically
checked.
This still contains the deprecated "arm,pl11x,tft-r0g0b0-pads" property,
because this is used by several DTs in the tree.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
[robh: make interrupts optional (not connected on Realview EB)]
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220506140533.3566431-9-andre.przywara@arm.com