net: marvell: prestera: devlink: add traps with DROP action

Add traps that have init_action being set to DROP.
Add 'trap_drop_counter_get' (devlink API) callback implementation,
that is used to get number of packets that have been dropped by the HW
(traps with action 'DROP').
Add new FW command CPU_CODE_COUNTERS_GET.

Signed-off-by: Oleksandr Mazur <oleksandr.mazur@plvision.eu>
Signed-off-by: David S. Miller <davem@davemloft.net>