[MMC] Prevent au1xmmc.c breakage on non-Au1200 Alchemy

The driver is selectable on other than Au1200 Alchemy systems but won't
build nor work - there is no MMC hw.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>