bridge: mrp: Implement the MRP Interconnect API

Thie patch adds support for MRP Interconnect. Similar with the MRP ring,
if the HW can't generate MRP_InTest frames, then the SW will try to
generate them. And if also the SW fails to generate the frames then an
error is return to userspace.

The forwarding/termination of MRP_In frames is happening in the kernel
and is done by MRP instances.

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>