[MMC] au1xmmc: Fix compilation error by using platform_driver

drivers/mmc/au1xmmc.c currently doesn't compile; it needs to be
converted to use platform_driver.  I cannot test this change because
of lack of hardware but I followed the drivers this one is based on,
and the code is certainly not worse than before.

drivers/mmc/au1xmmc.c: At top level:
drivers/mmc/au1xmmc.c:1002: error: ‘platform_bus_type’ undeclared here (not in a function)
make[2]: *** [drivers/mmc/au1xmmc.o] Error 1

Signed-off-by: Martin Michlmayr <tbm@cyrius.com>
Acked-by: Jordan Crouse <jordan.crouse@amd.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>