Platform/ARM/SgiPkg: Obtain rd-e1-edge platform core count from PCD

The number of processing elements is hardcoded in the MADT ACPI table
of the RD-E1-Edge platform. Remove the hardcoded value and obtain the
core count on platform from PCDs instead.

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