mmc: select REGMAP_MMIO with MMC_LOONGSON2

COMPILE_TEST with MMC_LOONGSON2 failed to link due to
undeclared dependency:

ERROR: modpost: "__devm_regmap_init_mmio_clk"
[drivers/mmc/host/loongson2-mmc.ko] undefined!

Fixes: 21157720 ("mmc: loongson2: Add Loongson-2K SD/SDIO controller driver")

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Suggested-by: Binbin Zhou <zhoubinbin@loongson.cn>
Signed-off-by: Mikko Rapeli <mikko.rapeli@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>