net: dsa: sja1105: Add P/Q/R/S management route support via dynamic interface

Management routes are one-shot FDB rules installed on the CPU port for
sending link-local traffic.  They are a prerequisite for STP, PTP etc to
work.

Also make a note that removing a management route was not supported on
the previous generation of switches.

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