net: dsa: add hook to determine whether EEE is supported

Add a hook to determine whether the switch supports EEE. This will
return false if the switch does not, or true if it does. If the
method is not implemented, we assume (currently) that the switch
supports EEE.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://patch.msgid.link/E1tL144-006cZD-El@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>