mlxsw: core: Consolidate trap groups to a single event group

For event traps which are used in core, avoid having a separate trap
group for each event. Instead of that introduce a single core event trap
group and use it for all event traps.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>