phy: Add MIPI D-PHY mode

MIPI D-PHY is a MIPI standard meant mostly for display and cameras in
embedded systems. Add a mode for it.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>