igc: Add MQPRIO offload support

Add support for offloading MQPRIO. The hardware has four priorities as well
as four queues. Each queue must be a assigned with a unique priority.

However, the priorities are only considered in TSN Tx mode. There are two
TSN Tx modes. In case of MQPRIO the Qbv capability is not required.
Therefore, use the legacy TSN Tx mode, which performs strict priority
arbitration.

Example for mqprio with hardware offload:

|tc qdisc replace dev ${INTERFACE} handle 100 parent root mqprio num_tc 4 \
|   map 0 0 0 0 0 1 2 3 0 0 0 0 0 0 0 0 \
|   queues 1@0 1@1 1@2 1@3 \
|   hw 1

The mqprio Qdisc also allows to configure the `preemptible_tcs'. However,
frame preemption is not supported yet.

Tested on Intel i225 and implemented by following data sheet section 7.5.2,
Transmit Scheduling.

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Wojciech Drewek <wojciech.drewek@intel.com>
Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Mor Bar-Gabay <morx.bar.gabay@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>