Add a driver for Cadence DPI -> DSI bridge.

This driver only support a subset of Cadence DSI bridge capabilities.

This driver has been tested/debugged in a simulated environment which
explains why some of the features are missing.  Here is a
non-exhaustive list of missing features:
 * burst mode
 * DPHY init/configuration steps
 * support for additional input interfaces (SDI input)

DSI commands and non-burst video mode have been tested.
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Acked-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180421070846.10330-1-boris.brezillon@bootlin.com