drm/omap: dsi: introduce mipi_dsi_host

This moves from custom platform driver infrastructure to mipi_dsi_host
and mipi_dsi_device. Note, that this is a graduate step and the driver
only uses the devices types and transfer function, but not yet the new
device binding style or drm_panel.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-13-tomi.valkeinen@ti.com