net: dsa: bcm_sf2: provide own phylink MAC operations

Convert bcm_sf2 to provide its own phylink MAC operations, thus
avoiding the shim layer in DSA's port.c

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Acked-by: Florian Fainelli <florian.fainelli@broadcom.com>
Tested-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://lore.kernel.org/r/E1rwfu3-00752s-On@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>