mlxsw: spectrum_policer: Add policer core

Add common code to handle all policer-related functionality in mlxsw.
Currently, only policer for policy engines are supported, but it in the
future more policer families will be added such as CPU (trap) policers
and storm control policers.

The API allows different modules to add / delete policers and read their
drop counter.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>