net: dsa: b53: Move Broadcom header setup to b53

The code to enable Broadcom tags/headers is largely switch independent,
and in preparation for enabling it for multiple devices with b53, move
the code we have in bcm_sf2.c to b53_common.c

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>