net: dsa: b53: Move EEE functions to b53

Move the bcm_sf2 EEE-related functions to the b53 driver because this is shared
code amongst Gigabit capable switch, only 5325 and 5365 are too old to support
that.

Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>