net: dsa: sja1105: Add a high-level overview of the dynamic config interface

When trying to add support for LOCKEDS (static FDB entries) on SJA1105
P/Q/R/S, at first I didn't remember how the abstraction I created
worked, and actually thought it works by mistake.

To avoid other people staring at the code and not making much sense out
of it, add some comments at the top of the file.

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