fbdev: sh-mobile: implement MIPI DSI runtime PM support

On SH-Mobile platforms using runtime PM with the MIPI DSI driver switches the
DSI Tx link clock on PM events.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>