net: dsa: bcm_sf2: Remove duplicate code

Now that we are using b53_common for most VLAN, FDB and bridge
operations, delete all the redundant code that we had in bcm_sf2.c to
keep only the integration specific logic that we have to deal with:
power management, link management and the external interfaces (RGMII,
MDIO).

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