l2mc/ipmc support multi-NPU

[zhichuanhe]

I have discussed these revision with @MaruthamuthuP through email,the detail information refer to http://lists.opencompute.org/pipermail/opencompute-sai/2018-May/subject.html.

In multiple NPU scenario,the ingress NPU need to allocate a H/W resource called the IPMC index which will be indexed into table to give the ports list on which a multicast packet should go out, the other egress NPUs will use the IPMC index to read H/W table directly without going through the lookup process based on packet's (S,G/x,G).From the point of view of the upper users(SONIC),the IPMC member are sets of router interface,and the same IPMC member can share a group,but from the point of view of NPU,finally,the group member need to convert to port list,so the relationship between IPMC Group and IPMC entry must be 1:1