arch/tile: tilegx PCI root complex support

This change implements PCIe root complex support for tilegx using
the kernel support layer for accessing the TRIO hardware shim.

Reviewed-by: Bjorn Helgaas <bhelgaas@google.com> [changes in 07487f3]
Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>