drm/pl111: Add handling of Versatile platforms

The ARM reference designs in the Versatile family: Integrator,
Versatile and RealView can make use of the new DRM driver as well.
We just need to create a bit of platform-specific code for them
that we isolate to its own file.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20170908124709.4758-7-linus.walleij@linaro.org