dt-bindings: mmc: Add Loongson-2K SD/SDIO/eMMC controller binding

Add the Loongson-2K SoC's SD/SDIO/eMMC controller binding with DT schema
format using json-schema.

Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Huacai Chen <chenhuacai@loongson.cn>
Link: https://lore.kernel.org/r/949c55be120a806ea3d74b47fa2cc96ced2905fc.1750765495.git.zhoubinbin@loongson.cn


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