Silicon/SynQuacer/PlatformDxe: move EMMC SSDT handling to core routine

In preparation of adding support for describing the presence of OP-TEE
via a SSDT ACPI table, refactor the existing code so we will be able to
reuse it more easily.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>