net/mlx5e: Add ToS (DSCP) header rewrite support

Add support for rewriting of DSCP part of ToS field.
Next commands, for example, can be used to offload rewrite action:

OVS:
 $ ovs-ofctl add-flow ovs-sriov "ip, in_port=REP, \
       actions=mod_nw_tos:68, output:NIC"

iproute2 (used retain mask, as tc command rewrite whole ToS field):
 $ tc filter add dev REP ingress protocol ip prio 1 flower skip_sw \
       ip_proto icmp action pedit munge ip tos set 68 retain 0xfc pipe \
       action mirred egress redirect dev NIC

Signed-off-by: Dmytro Linkin <dmitrolin@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>