mmc: loongson2: Add Loongson-2K SD/SDIO controller driver

mmc: loongson2: Add Loongson-2K SD/SDIO controller driver

The MMC controllers on the Loongson-2K series CPUs are similar,
except for the interface characteristics and the use of DMA controllers.

This patch describes the MMC controllers on the Loongson-2K0500/2K1000,
with the distinguishing feature being the use of an externally shared
APBDMA engine.

Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Reviewed-by: Huacai Chen <chenhuacai@loongson.cn>
Link: https://lore.kernel.org/r/c0a9f0c0279d8e09165c6e2d694b0c35f7fc7e31.1750765495.git.zhoubinbin@loongson.cn
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>