MdeModulePkg: PcdAcpiExposedTableVersions default for Arm64 platforms

Set the default value of PcdAcpiExposedTableVersions for Aarch64
platforms to 0x20. Previously, the default was set to 0x3E for all
platforms. The new value removes ACPI 1.0b compatability, which forces
the use of XSDT 64-bit pointer, as required by Arm SBBR specification.
This also resolves an error reported by acpiview command, as seen on
the RPi (see https://github.com/pftf/RPi4/issues/25

).

Signed-off-by: Samer El-Haj-Mahmoud <samer@elhajmahmoud.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>
Acked-by: Hao A Wu <hao.a.wu@intel.com>