MIPS: Alchemy: only build mmc support helpers if au1xmmc is enabled

While commit d4a5c59a ("mmc: au1xmmc: force non-modular build and
remove symbol_get usage") to be built in, it can still build a kernel
without MMC support and thuse no mmc_detect_change symbol at all.

Add ifdefs to build the mmc support code in the alchemy arch code
conditional on mmc support.

Fixes: d4a5c59a ("mmc: au1xmmc: force non-modular build and remove symbol_get usage")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>