Add per-port vlan support for the AR8327.

All the per-port support is really doing is applying a port visibility
mask to each of the switchports. Everything still look like a single
portgroup (vlan id 1), but the per-port visibility mask is modified.

Whilst I'm here, also add some initial dot1q support - the pvid stuff
is doing the right thing, but it's not useful without the rest of
the VLAN table programming.

It's enough for me to be able to use the LAN/WAN port distinction
on the AP135, where there isn't (for now!) a dedicated PHY for the
"WAN" port.

Tested:

• AP135, QCA9558 SoC + AR8327 switch