mlxsw: Add option to choose trap group

Currently, we set the trap group to pre-determined option, based on whether
it is an rx or event trap.
This commit adds a possibility to chose the trap group, so it can be set
to different values in the following patches.

Signed-off-by: Nogah Frankel <nogahf@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>