net: dsa: allow XAUI phy interface mode

XGMII is a 32-bit bus plus two clock signals per direction.  XAUI is
four serial lanes per direction.  The 88e6190 supports XAUI but not
XGMII as it doesn't have enough pins.  The same is true of 88e6176.

Match on PHY_INTERFACE_MODE_XAUI for the XAUI port type, but keep
accepting XGMII for backwards compatibility.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>