mlxsw: spectrum: Add IEEE 802.1Qaz ETS support

Implement the appropriate DCB ops and allow a user to configure:
	* Priority to traffic class (TC) mapping with a total of 8
	  supported TCs
	* Transmission selection algorithm (TSA) for each TC and the
	  corresponding weights in case of weighted round robin (WRR)

As previously explained, we treat the priority group (PG) buffer in the
port's headroom as the ingress counterpart of the egress TC. Therefore,
when a certain priority to TC mapping is configured, we also configure
the port's headroom buffer.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>