sh-mobile: add support for displays, connected over the MIPI bus

Some SH-mobile SoCs have a MIPI DSI controller, that can be used to connect
MIPI displays to LCDC. This patch adds a platform driver for SH-mobile MIPI DSI
unit. It uses existing hooks in the sh_mobile_lcdcfb.c driver for display
activation and deactivation.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Tested-by: Damian Hobson-Garcia <dhobsong@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>