net: dsa: mv88e6xxx: Implement interrupt support.

The switch can have up to two interrupt controllers. One of these
contains the interrupts from the integrated PHYs, so is useful to
export. The Marvell PHY driver can then be used in interrupt mode,
rather than polling, speeding up PHY handling and reducing load on the
MDIO bus.

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