net: dsa: bcm_sf2: implement GPHY power down

Implement the power on/off recommended procedure for the Single GPHY we
have on our Starfighter 2 switch. In order to make sure we get proper
LED link/activity signaling during suspend, switch the link indication
from the Switch/MAC to the PHY.

Finally, since the GPHY needs to be reset to be put in low power mode,
we will loose any context applied to it: workarounds, EEE etc.. so we
need to call phy_init_hw() to get our fixups re-applied successfully.

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