arch: tile: gxio: Export symbols for module using in 'mpipe.c'

'gxio_mpipe_adjust_timestamp', 'gxio_mpipe_link_instance',
'gxio_mpipe_get_timestamp', and 'gxio_mpipe_set_timestamp' may be use by
other tile modules, so export them.

The related error (with allmodconfig under tile):

    MODPOST 4002 modules
  ERROR: "gxio_mpipe_link_instance" [drivers/net/ethernet/tile/tile_net.ko] undefined!
  ERROR: "gxio_mpipe_get_timestamp" [drivers/net/ethernet/tile/tile_net.ko] undefined!
  ERROR: "gxio_mpipe_set_timestamp" [drivers/net/ethernet/tile/tile_net.ko] undefined!
  ERROR: "gxio_mpipe_adjust_timestamp" [drivers/net/ethernet/tile/tile_net.ko] undefined!

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>