fbdev: sh_mipi_dsi: support different register layouts

The register layout of the MIPI DSI unit on sh-mobile SoCs differ. Add platform
parameters to support such variations.

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