dt-bindings: mmc: samsung,exynos-dw-mshc: document Exynos5420 SMU

Document the compatible for Samsung Exynos5420 SoC Synopsys Designware
MSHC with SMU, already used in DTS and driver.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220605163710.144210-5-krzysztof.kozlowski@linaro.org


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