Silicon/Qemu: Use PCDs to AcpiTables lib

The ACPI Tables providing library AcpiTables.inf uses
a lot of information that is available in the form of PCDs
for differnt platforms.

This patch replaces hardcoded values describing the PCIe,
AHCI, EHCI, and Serial with the appropriate PCDs.

Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Tanmay Jagdale <tanmay.jagdale@linaro.org>
Signed-off-by: Tomas Pilar <tomas@nuviateam.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>