drm: Add MIPI DSI bus support

MIPI DSI bus allows to model DSI hosts and DSI peripherals using the
Linux driver model. DSI hosts are registered by the DSI host drivers.
During registration DSI peripherals will be created from the children
of the DSI host's device tree node. Support for registration from
board-setup code will be added later when needed.

DSI hosts expose operations which can be used by DSI peripheral drivers
to access associated devices.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>