sh: allow platforms to specify SD-card supported voltages

Boards can have different supplied voltages on different SD card slots. This
information has to be passed down to the SD/MMC driver.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Ian Molton <ian@mnementh.co.uk>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>