net: dsa: sja1105: offload the Credit-Based Shaper qdisc

SJA1105, being AVB/TSN switches, provide hardware assist for the
Credit-Based Shaper as described in the IEEE 8021Q-2018 document.

First generation has 10 shapers, freely assignable to any of the 4
external ports and 8 traffic classes, and second generation has 16
shapers.

The Credit-Based Shaper tables are accessed through the dynamic
reconfiguration interface, so we have to restore them manually after a
switch reset. The tables are backed up by the static config only on
P/Q/R/S, and we don't want to add custom code only for that family,
since the procedure that is in place now works for both.

Tested with the following commands:

data_rate_kbps=67000
port_transmit_rate_kbps=1000000
idleslope=$data_rate_kbps
sendslope=$(($idleslope - $port_transmit_rate_kbps))
locredit=$((-0x80000000))
hicredit=$((0x7fffffff))
tc qdisc add dev swp2 root handle 1: mqprio hw 0 num_tc 8 \
        map 0 1 2 3 4 5 6 7 \
        queues 1@0 1@1 1@2 1@3 1@4 1@5 1@6 1@7
tc qdisc replace dev swp2 parent 1:1 cbs \
        idleslope $idleslope \
        sendslope $sendslope \
        hicredit $hicredit \
        locredit $locredit \
        offload 1

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