Platform/Sgi: Add helper macros for SSDT and Mcfg tables

Add a helper macro to define the PCI root complex ECAM region properties
for Mcfg table, and another macro to create PCI Root Complex resource in
the SSDT table.

Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: I05fb0b54f4ec363f0269a10037f3553d73aa4951