net: mscc: ocelot: add support for mqprio offload

This doesn't apply anything to hardware and in general doesn't do
anything that the software variant doesn't do, except for checking that
there isn't more than 1 TXQ per TC (TXQs for a DSA switch are a dubious
concept anyway). The reason we add this is to be able to parse one more
field added to struct tc_mqprio_qopt_offload, namely preemptible_tcs.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Ferenc Fejes <fejes@inf.elte.hu>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>