bridge: mrp: Extend br_mrp for MRP interconnect

This patch extends the 'struct br_mrp' to contain information regarding
the MRP interconnect. It contains the following:
- the interconnect port 'i_port', which is NULL if the node doesn't have
  a interconnect role
- the interconnect id, which is similar with the ring id, but this field
  is also part of the MRP_InTest frames.
- the interconnect role, which can be MIM or MIC.
- the interconnect state, which can be open or closed.
- the interconnect delayed_work for sending MRP_InTest frames and check
  for lost of continuity.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>