net: mscc: ocelot: don't hardcode the number of the CPU port

VSC7514 is a 10-port switch with 2 extra "CPU ports" (targets in the
queuing subsystem for terminating traffic locally).

There are 2 issues with hardcoding the CPU port as #10:
- It is not clear which snippets of the code are configuring something
  for one of the CPU ports, and which snippets are just doing something
  related to the number of physical ports.
- Actually any physical port can act as a CPU port connected to an
  external CPU (in addition to the local CPU). This is called NPI mode
  (Node Processor Interface) and is the way that the 6-port VSC9959
  (Felix) switch is integrated inside NXP LS1028A (the "local management
  CPU" functionality is not used there).

This patch makes it clear that the ocelot_bridge_stp_state_set function
operates on the CPU port (by making it an implicit member of the
bridging domain), and at the same time adds logic for the NPI port (aka
a physical port) to play the role of a CPU port (it shouldn't be part of
bridge_fwd_mask, as it's not explicitly enslaved to a bridge).

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>