net: dsa: sja1105: Add support for Spanning Tree Protocol

While not explicitly documented as supported in UM10944, compliance with
the STP states can be obtained by manipulating 3 settings at the
(per-port) MAC config level: dynamic learning, inhibiting reception of
regular traffic, and inhibiting transmission of regular traffic.

In all these modes, transmission and reception of special BPDU frames
from the stack is still enabled (not inhibited by the MAC-level
settings).

On ingress, BPDUs are classified by the MAC filter as link-local
(01-80-C2-00-00-00) and forwarded to the CPU port.  This mechanism works
under all conditions (even without the custom 802.1Q tagging) because
the switch hardware inserts the source port and switch ID into bytes 4
and 5 of the MAC-filtered frames. Then the DSA .rcv handler needs to put
back zeroes into the MAC address after decoding the source port
information.

On egress, BPDUs are transmitted using management routes from the xmit
worker thread. Again this does not require switch tagging, as the switch
port is programmed through SPI to hold a temporary (single-fire) route
for a frame with the programmed destination MAC (01-80-C2-00-00-00).

STP is activated using the following commands and was tested by
connecting two front-panel ports together and noticing that switching
loops were prevented (one port remains in the blocking state):

$ ip link add name br0 type bridge stp_state 1 && ip link set br0 up
$ for eth in $(ls /sys/devices/platform/soc/2100000.spi/spi_master/spi0/spi0.1/net/);
  do ip link set ${eth} master br0 && ip link set ${eth} up; done

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>