drm/mediatek: Add DRM Driver for Mediatek SoC MT8173.

This patch adds an initial DRM driver for the Mediatek MT8173 DISP
subsystem. It currently supports two fixed output streams from the
OVL0/OVL1 sources to the DSI0/DPI0 sinks, respectively.

Signed-off-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: YT Shen <yt.shen@mediatek.com>
Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Signed-off-by: Mao Huang <littlecvr@chromium.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>