mfd: Add power control platform data to SDHI driver

This patch adds platform data with a function for power
control to the SDHI driver. The idea is that board specific
code can provide their own functions so power can be enabled
and disabled for the sd-cards.

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