MdeModulePkg: introduce SD/MMC override protocol

Many ARM based SoCs have integrated SDHCI controllers, and often,
these implementations deviate in subtle ways from the pertinent
specifications. On the one hand, these deviations are quite easy
to work around, but on the other hand, having a collection of SoC
specific workarounds in the generic driver stack is undesirable.

So let's introduce an optional SD/MMC override protocol that we
can invoke at the appropriate moments in the device initialization.
That way, the workaround itself remains platform specific, but we
can still use the generic driver stack on such platforms.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>
​Reviewed-by: Star Zeng <star.zeng@intel.com>