net: dsa: mv88e6xxx: add mv88e6352_g2_scratch_port_has_serdes()

Read the hardware configuration to determine which port is attached
to the serdes.

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