net: dsa: bcm_sf2: add support for controlling EEE

When EEE is enabled, negotiate this feature with the PHY and make sure
that the capability checking, local EEE advertisement, link partner EEE
advertisement and auto-negotiation resolution returned by phy_init_eee()
is positive, and enable EEE at the switch level.

While querying the current EEE settings, verify the low-power indication
and indicate its status.

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