Platform/ARM/SgiPkg: Refactor GIC related ACPI helper macros

Remove the duplication of the ACPI helper macros related to GIC
distributor, redistributor and ITS from each platform's MADT ACPI
table. Let these macros be defined in the common SgiAcpiHeader.h
file.

Signed-off-by: Aditya Angadi <aditya.angadi@arm.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@arm.com>