drm: rockchip: Add basic drm driver

This patch adds the basic structure of a DRM Driver for Rockchip Socs.

Signed-off-by: Mark Yao <mark.yao@rock-chips.com>
Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Rob Clark <robdclark@gmail.com>