net: dsa: mv88e6xxx: Add watchdog interrupt handler

The switch contains a watchdog looking for issues with the internal
gubbins of the switch. Hook the interrupt the watchdog triggers and
log the value of the control register indicating why the watchdog
fired. The watchdog can only be cleared with a switch reset, which
will destroy the current configuration. Rather than doing this, just
disable the interrupt.

The mv88e6390 family has different watchdog registers. So use an ops
structure, so support for the mv88e6390 family can be added later.

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