Platform/RPi4/ConfigDxe: Replace SD Host with eMMC2 SDHCI option

SD Host is not usable on Pi 4 so replace it with an eMMC2 option.

Note that is a menu only change at this stage, since we don't have
eMMC support finalized.

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>