net: mscc: ocelot: Add support for MRP

Add basic support for MRP. The HW will just trap all MRP frames on the
ring ports to CPU and allow the SW to process them. In this way it is
possible to for this node to behave both as MRM and MRC.

Current limitations are:
- it doesn't support Interconnect roles.
- it supports only a single ring.
- the HW should be able to do forwarding of MRP Test frames so the SW
  will not need to do this. So it would be able to have the role MRC
  without SW support.

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