[MMC] ios for mmc chip select

Adds a new ios for setting the chip select pin on MMC cards. Needed on
SD controllers which use this pin for other things and therefore cannot
have it pulled high at all times.

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