net: airoha: Add sched ETS offload support

Introduce support for ETS Qdisc offload available on the Airoha EN7581
ethernet controller. In order to be effective, ETS Qdisc must configured
as leaf of a HTB Qdisc (HTB Qdisc offload will be added in the following
patch). ETS Qdisc available on EN7581 ethernet controller supports at
most 8 concurrent bands (QoS queues). We can enable an ETS Qdisc for
each available QoS channel.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>