Platform/Sgi: enable dynamic acpi table generation for rdn2cfg3

RdN2Cfg3 configuration is a new variant of RD platform. To enable
dynamic generation of PCIe SSDT, MCFG and IORT ACPI tables, add the
configuration PCDs required by the library in the platform
description file.

Signed-off-by: Nishant Sharma <nishant.sharma@arm.com>
Change-Id: I12430705e7b9599e802010004ed6386557e78d62