drm: Add support of ARC PGU display controller

ARC PGU could be found on some development boards from Synopsys.
This is a simple byte streamer that reads data from a framebuffer
and sends data to the single encoder.

Signed-off-by: Carlos Palminha <palminha@synopsys.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-snps-arc@lists.infradead.org