SgiPkg/RdN1EdgeX2: Add missing reference to PcdChipCount

Commit e8fe2026 (“Platform/ARM/SgiPkg: Use chip count constant on
rdn1edgex2 platformâ€) used the PcdChipCount constant but did not declare
its use in the ACPI table module. Fix this by listing it in the list of
PCDs to be looked up.

Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Reviewed-by: Thomas Abraham <thomas.abraham@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>