tile_net: Always enable PTP clock support on TILE-Gx

All other net drivers with PTP support enable it unconditionally.
Make tile_net consistent with this.

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