tile: support multiple mPIPE shims in tilegx network driver

The initial driver support was for a single mPIPE shim on the chip
(as is the case for the Gx36 hardware).  The Gx72 chip has two mPIPE
shims, so we extend the driver to handle that case.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
Signed-off-by: David S. Miller <davem@davemloft.net>