mlxsw: spectrum: Support ieee_setapp, ieee_delapp

The APP TLVs are used for communicating priority-to-protocol ID maps for
a given netdevice. Support the following APP TLVs:

- DSCP (selector 5) to configure priority-to-DSCP code point maps. Use
  these maps to configure packet priority on ingress, and DSCP code
  point rewrite on egress.

- Default priority (selector 1, PID 0) to configure priority for the
  DSCP code points that don't have one assigned by the DSCP selector. In
  future this could also be used for assigning default port priority
  when a packet arrives without DSCP tagging.

Besides setting up the maps themselves, also configure port trust level
and rewrite bits.

Port trust level determines whether, for a packet arriving through a
certain port, the priority should be determined based on PCP or DSCP
header fields. So far, mlxsw kept the device default of trust-PCP. Now,
as soon as the first DSCP APP TLV is configured, switch to trust-DSCP.
Only when all DSCP APP TLVs are removed, switch back to trust-PCP again.
Note that the default priority APP TLV doesn't impact the trust level
configuration.

Rewrite bits determine whether DSCP and PCP fields of egressing packets
should be updated according to switch priority. When port trust is
switched to DSCP, enable rewrite of DSCP field.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>