dsa: mv88e6xxx: Enable/Disable SERDES on port enable/disable

Implement the port enable/disable callbacks, which enable/disable the
SERDES interfaces, if applicable. This should save a bit of
power/heat.

We also need to enable SERDES on CPU and DSA ports, so keep the
existing call to the op, but make it conditional.

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