drm/dsi: Resolve MIPI DSI device from phandle

Add a function, of_find_mipi_dsi_device_by_node(), that can be used to
resolve a phandle to a MIPI DSI device.

Acked-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>