net: airoha: Add sched HTB offload support

Introduce support for HTB Qdisc offload available in the Airoha EN7581
ethernet controller. EN7581 can offload only one level of HTB leafs.
Each HTB leaf represents a QoS channel supported by EN7581 SoC.
The typical use-case is creating a HTB leaf for QoS channel to rate
limit the egress traffic and attach an ETS Qdisc to each HTB leaf in
order to enforce traffic prioritization.

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