mlxsw: core_acl_flex_actions: Add mirror sampler action

Add core functionality required to support mirror sampler action in the
policy engine. The switch driver (e.g., 'mlxsw_spectrum') is required to
implement the sampler_add() / sampler_del() callbacks that perform the
necessary configuration before the sampler action can be installed. The
next patch will implement it for Spectrum-{2,3}, while Spectrum-1 will
return an error, given it is not supported.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>