net: pcs: xpcs: Add Pause Mode support for SGMII and 2500BaseX

SGMII/2500BaseX supports Pause frame as defined in the IEEE802.3x
Flow Control standardization.

Add this as a supported feature under the xpcs_sgmii_features struct.

Cc: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Wong Vee Khee <vee.khee.wong@linux.intel.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>