bridge: uapi: mrp: Add mrp attributes.

Add new nested netlink attribute to configure the MRP. These attributes are used
by the userspace to add/delete/configure MRP instances and by the kernel to
notify the userspace when the MRP ring gets open/closed. MRP nested attribute
has the following attributes:

IFLA_BRIDGE_MRP_INSTANCE - the parameter type is br_mrp_instance which contains
  the instance id, and the ifindex of the two ports. The ports can't be part of
  multiple instances. This is used to create/delete MRP instances.

IFLA_BRIDGE_MRP_PORT_STATE - the parameter type is u32. Which can be forwarding,
  blocking or disabled.

IFLA_BRIDGE_MRP_PORT_ROLE - the parameter type is br_mrp_port_role which
  contains the instance id and the role. The role can be primary or secondary.

IFLA_BRIDGE_MRP_RING_STATE - the parameter type is br_mrp_ring_state which
  contains the instance id and the state. The state can be open or closed.

IFLA_BRIDGE_MRP_RING_ROLE - the parameter type is br_mrp_ring_role which
  contains the instance id and the ring role. The role can be MRM or MRC.

IFLA_BRIDGE_MRP_START_TEST - the parameter type is br_mrp_start_test which
  contains the instance id, the interval at which to send the MRP_Test frames,
  how many test frames can be missed before declaring the ring open and the
  period which represent for how long to send the test frames.

Also add the file include/uapi/linux/mrp_bridge.h which defines all the types
used by MRP that are also needed by the userpace.

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>