Platform/Sgi: Add Mcfg table for Rd-N2

RD-N2 platform contains PCIe Root Bridge with SMMU in the path as
part of the IO virtualization block. To allow the OSPM to discover
and enumerate PCIe topology, add the Mcfg table.

Signed-off-by: Vivek Gautam <vivek.gautam@arm.com>
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: I095114b2bb894b969af0b03aa330e046246f2562