net: bcmgenet: Add support for PHY-based Wake-on-LAN

If available, interrogate the PHY to find out whether we can use it for
Wake-on-LAN. This can be a more power efficient way of implementing
that feature, especially when the MAC is powered off in low power
states.

Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>