net: dsa: mv88e6xxx: add VLAN Get Next support

Implement the port_pvid_get and vlan_getnext driver functions required
to dump VLAN entries from the hardware, with the VTU Get Next operation.

Some functions and structure will be shared with STU operations, since
their table format are similar (e.g. STU data entries are accessible
with the same registers as VTU entries, except with an offset of 2).

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>