tile PCI RC: support more MSI-X interrupt vectors

To support PCIe devices with higher number of MSI-X interrupt vectors,
e.g. 16 for the LSI RAID card, enhance the Gx RC stack to provide more
MSI-X vectors by using the TRIO Scatter Queues, which provide 8 more
vectors in addition to ~10 from the Map Mem regions.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>