mlxsw: spectrum: qdiscs: Support PRIO qdisc offload

Add support for offloading PRIO qdisc as root qdisc.
The support is for up to 8 bands.
Routed packets priority is determined by the DSCP field with the default
translations. Bridged packets priority is determined by the PCP field, if
exist, otherwise it is set to 0.
Since both options have only priorities 0-7, higher priorities mapping are
being ignored.

Signed-off-by: Nogah Frankel <nogahf@mellanox.com>
Reviewed-by: Yuval Mintz <yuvalm@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>