dt-bindings: display: add devicetree bindings for pxa300-gcu

This patch adds the binding documentation for the pxa300 gcu.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>