net: bridge: Add port attribute IFLA_BRPORT_MRP_RING_OPEN

This patch adds a new port attribute, IFLA_BRPORT_MRP_RING_OPEN, which allows
to notify the userspace when the port lost the continuite of MRP frames.

This attribute is set by kernel whenever the SW or HW detects that the ring is
being open or closed.

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