net: dsa: hellcreek: Report VLAN table occupancy

The VLAN membership configuration is cached in software already. So, it can be
reported via devlink. Add support for it:

|root@tsn:~# devlink resource show platform/ff240000.switch
|platform/ff240000.switch:
|  name VLAN size 4096 occ 4 unit entry dpipe_tables none

Signed-off-by: Kurt Kanzenbach <kurt@kmk-computers.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>