arm64: dts: exynos: align MMC node name with dtschema

The node names should be generic and MMC controller dtschema expects
"mmc".

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>
Link: https://lore.kernel.org/r/20220605163710.144210-3-krzysztof.kozlowski@linaro.org