net: dsa: sja1105: Make room for P/Q/R/S FDB operations

The DSA callbacks were written with the E/T (first generation) in mind,
which is quite different.

For P/Q/R/S completely new implementations need to be provided, which
are held as function pointers in the priv->info structure.  We are
taking a slightly roundabout way for this (a function from
sja1105_main.c reads a structure defined in sja1105_spi.c that
points to a function defined in sja1105_main.c), but it is what it is.

The FDB dump callback works for both families, hence no function pointer
for that.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>