net: ethernet: ti: am65-cpsw: add mqprio qdisc offload in channel mode

This patch add MQPRIO Qdisk offload in full 'channel' mode which allows not
only setup pro:tc mapping, but also configure TX shapers on external port
fifos. The K3 CPSW MQPRIO Qdisk offload is expected to work with
VALN/priority tagged packets and not-tagged packets have to be mapped only
to TC0.

- TX traffic classes must be rated starting from TC that has highest
priority and with no gaps
- Traffic classes are used starting from 0, that has highest priority
- min_rate defines Committed Information Rate (guaranteed)
- max_rate defines Excess Information Rate (non guaranteed) and offloaded
as (max_rate[i] - tcX_min_rate[i])
- VALN/priority tagged packets mapped to TC0 will exit switch with VALN tag
priority 0

The configuration example:
 ethtool -L eth1 tx 5
 ethtool --set-priv-flags eth1 p0-rx-ptype-rrobin off

 tc qdisc add dev eth1 parent root handle 100: mqprio num_tc 3 \
 map 0 0 1 2 0 0 0 0 0 0 0 0 0 0 0 0 \
 queues 1@0 1@1 1@2 h...