net: mvpp2: cycle comphy to power it down

Presently, at boot time, the comphys are enabled. For firmware
compatibility reasons, the comphy driver does not power down the
comphys at boot. Consequently, the ethernet comphys are left active
until the network interfaces are brought through an up/down cycle.

If the port is never used, the port wastes power needlessly. Arrange
for the ethernet comphys to be cycled by the mvpp2 driver as if the
interface went through an up/down cycle during driver probe, thereby
powering them down.

This saves:
  270mW per 10G SFP+ port on the Macchiatobin Single Shot (eth0/eth1)
  370mW per 10G PHY port on the Macchiatobin Double Shot (eth0/eth1)
  160mW on the SFP port on either Macchiatobin flavour (eth3)

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Acked-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>