Commit 802.1q configuration support for the AR8327.

This is slightly different to the other switches - the VLAN table
(VTU) programs in the vlan port mapping /and/ the port config
(tagged, untagged, passthrough, any.)

So:

• Add VTU operations to program the VTU (vlan table)
• abstract out the mirror-disable function so it's .. well, a function.
• setup the port to have a dot1q configuration for dot1q - the port security is VLAN (not per-port VLAN) and requires an entry in the VLAN table;
• add set_dot1q / get_dot1q to program the VLAN table;
• since the tagged/untagged ports are now programmed into the VTU, rather than global - plumb the ports /and/ untagged ports bitmaps through the arswitch API.

Tested:

• AP135 - QCA9558 SoC + AR8327N switch