net: dsa: hellcreek: Report FDB table occupancy

Report the FDB table size and occupancy via devlink. The actual size depends on
the used Hellcreek version:

|root@tsn:~# devlink resource show platform/ff240000.switch
|platform/ff240000.switch:
|  name VLAN size 4096 occ 2 unit entry dpipe_tables none
|  name FDB size 256 occ 6 unit entry dpipe_tables none

Suggested-by: Florian Fainelli <f.fainelli@gmail.com>
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>