drm: Add DT bindings documentation for ARC PGU display controller

This add DT bindings documentation for ARC PGU display controller.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
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>
Cc: devicetree@vger.kernel.org
Cc: linux-snps-arc@lists.infradead.org
Acked-by: Rob Herring <robh@kernel.org>