net: dsa :mv88e6xxx: Disable unused ports

If the NO_CPU strap is set, the switch starts in 'dumb hub' mode, with
all ports enable. Ports which are then actively used are reconfigured
as required when the driver starts. However unused ports are left
alone. Change this to disable them, and turn off any SERDES
interface. This could save some power and so reduce the temperature a
bit.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>