Platform/ARM/SgiPkg: Use chip count constant on rdn1edgex2 platform

Use the PcdChipCount constant to define the maximum number of chips
included in the multi-chip package. In addition to this, remove the
hardcoded value of the chip count in the MADT ACPI table of RD-N1-EdgeX2
platform and replace it with the number of chips specified by
'PcdChipCount'.

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