net: dsa: lantiq_gswip: serialize access to the PCE registers

The GSWIP switch accesses various bridging layer tables (VLANs, FDBs,
forwarding rules) indirectly through PCE registers. These hardware
accesses are non-atomic, being comprised of several register reads and
writes.

These accesses are currently serialized by the rtnl_lock, but DSA is
changing its driver API and that lock will no longer be held when
calling ->port_fdb_add() and ->port_fdb_del().

So this driver needs to serialize the access to the PCE registers using
its own locking scheme. This patch adds that.

Note that the driver also uses the gswip_pce_load_microcode() function
to load a static configuration for the packet classification engine into
a table using the same registers. It is currently not protected, but
since that configuration is only done from the dsa_switch_ops :: setup
method, there is no risk of it being concurrent with other operations.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>