net: dsa: bcm_sf2: Prepare for adding CFP statistics

In preparation for adding CFP statistics, we will need to overlay the
standard B53 statistics, so create specific bcm_sf2_sw_* functions to
call into their b53_common.c counterpart.

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