mfd: Add SuperH Mobile SDHI platform driver

This patch adds an MFD driver for the SuperH Mobile SDHI
hardware block. At this point the driver simply wraps the
tmio-mmc driver with some clock code. In the future this
driver is the place to put SDHI specific hotplug code.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>