Platform/Sgi: allow support for multiple PCIe root bridge

For reference design platforms that support more than one PCIe host
bridge, update the PCIe host bridge library implementation to allow
support for upto four PCIe host bridges. PCDs are introduced to allow
platforms to specify the values for bus count and the various MMIO base
address and sizes.

The available PCIe resources are split equally between the various host
bridge instances in the system and macros have been introduced that
makes it easier to implment this split.

Signed-off-by: Vivek Gautam <vivek.gautam@arm.com>
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: I974f07f6442e85ce9dea9730bb3be4c955551965