net: dsa: mv88e6xxx: Unique SERDES interrupt names

Dynamically generate a unique SERDES interrupt name, based on the
device name and the port the SERDES is for. For example:

 95:          3  mv88e6xxx-g2   9 Edge      mv88e6xxx-0.2:00-serdes-9
 96:          0  mv88e6xxx-g2  10 Edge      mv88e6xxx-0.2:00-serdes-10

The 0.2:00 indicates the switch and -9 indicates port 9.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>