dt-bindings: display: convert Arm HDLCD to DT schema

The Arm HDLCD is a display controller that scans out a framebuffer and
hands a signal to a digital encoder to generate a DVI or HDMI signal.

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/20220506140533.3566431-10-andre.przywara@arm.com