drm/mediatek: Add DSI sub driver

This patch add a drm encoder/connector driver for the MIPI DSI function
block of the Mediatek display subsystem and a phy driver for the MIPI TX
D-PHY control module.

Signed-off-by: Jitao Shi <jitao.shi@mediatek.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>