arch/tile: provide kernel support for the tilegx TRIO shim

Provide kernel support for the tilegx "Transaction I/O" (TRIO) on-chip
hardware.  This hardware implements the PCIe interface for tilegx;
the driver changes to use TRIO for PCIe are in a subsequent commit.

The change is layered on top of the tilegx GXIO IORPC subsystem.

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