[MMC] sdhci: support for multiple voltages

The sdhci controllers can support up to three voltage levels.  Detect which
and report back to the MMC layer.

Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>