Add network device event for priority code point, PCP, changes.

When the PCP is changed for either a VLAN network interface or when
prio tagging is enabled for a regular ethernet network interface,
broadcast the IFNET_EVENT_PCP event so applications like ibcore can
update its GID tables accordingly.

MFC after: 3 days
Reviewed by: ae, kib
Differential Revision: https://reviews.freebsd.org/D15040
Sponsored by: Mellanox Technologies