mmc: loongson2: Add Loongson-2K2000 SD/SDIO/eMMC controller driver

This patch describes the two MMC controllers of the Loongson-2K2000 SoC,
one providing an eMMC interface and the other exporting an SD/SDIO
interface.

Compared to the Loongson-2K1000's MMC controllers, their internals are
similar, except that we use an internally exclusive DMA engine instead 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/1df46b976abd36003bd553ad8a039e5c97369df0.1750765495.git.zhoubinbin@loongson.cn


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>