drm/sun4i: Add Allwinner A31 MIPI-DSI controller support

Most of the Allwinner SoCs since the A31 share the same MIPI-DSI
controller.

While that controller is mostly undocumented, the code is out there and has
been cleaned up in order to be integrated into DRM. However, there's still
some dark areas that are a bit unclear about how the block exactly
operates.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ad9e6224fced87c0889ddd2765d1942610061f72.1522835818.git-series.maxime.ripard@bootlin.com