net/dcb: Add dscp to priority selector type

IEEE specification P802.1Qcd/D2.1 defines priority selector 5.
This APP TLV selector defines DSCP to priority map.
This patch defines such DSCP selector.

Signed-off-by: Huy Nguyen <huyn@mellanox.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>