dt-bindings: mmc: controller: Add max-sd-hs-hz property

Introduce a new optional device tree property max-sd-hs-hz to
limit the maximum frequency (in Hz) used for SD cards operating in
High-Speed (HS) mode due to any board electrical limitations.

Signed-off-by: Sarthak Garg <quic_sartgarg@quicinc.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>