Platform/Sgi: enable autogeneration of iort table

Add the support to generate IORT table from the PCIe topology
information passed from the previous boot stage. Add the support to
populate IORT table header and then populate DMA and SMMU instancs for
each HostBridge then populate the info for each RootComplex.

Signed-off-by: Nishant Sharma <nishant.sharma@arm.com>
Change-Id: I5e73a5a3cbac3c91c1df647e514f9888ef3a4466