drm/arm: Add support for Mali Display Processors

Add support for the new family of Display Processors from ARM Ltd.
This commit adds basic support for Mali DP500, DP550 and DP650
parts, with only the display engine being supported at the moment.

Cc: David Brown <David.Brown@arm.com>
Cc: Brian Starkey <Brian.Starkey@arm.com>

Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>