net: dsa: mv88e6xxx: Add number of internal PHYs

Add to the info structure the number of internal PHYs, if they generate
interrupts. Some of the older generations of switches have internal
PHYs, but no interrupt registers. In this case, set the count to zero.

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