net: bcmgenet: power up and down integrated GPHY when unused

Power up the GPHY while we are bringing-up the network interface, and
conversely, upon bring down, power the GPHY down. In order to avoid
creating hardware hazards, make sure that the GPHY gets powered on
during bcmgenet_open() prior to the UniMAC being reset as the UniMAC may
start creating activity towards the GPHY if we reverse the steps.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>