net: dsa: mv88e6xxx: Add support for SERDES on ports 2-8 for 6390X

The 6390X family has 8 SERDES interfaces. When ports 9 and 10 are not
using all their SERDES interfaces, the unused ones can be assigned to
ports 2-8. Add support for interrupts from SERDES interfaces connected
to these lower ports.

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