Documentation: drm: Add DT bindings for ARM HDLCD

Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>