bridge: mrp: Add br_mrp_fill_info

Add the function br_mrp_fill_info which populates the MRP attributes
regarding the status of each MRP instance.

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>