This adds support for the Armada 510 display subsystem found on the
Marvell Dove devices.  This IP is re-used across several different Marvell
SoCs with various tweaks, and this driver has been structured to allow
the other IPs to re-use the bulk of this code; further work in this area
is expected from interested parties.

This has been extensively tested on the SolidRun Cubox platform and
appears to work well there.

[airlied: update for api changes merged previous to this]