devlink: Add health get command

Add devlink health get command to provide reporter/s data for user space.
Add the ability to get data per reporter or dump data from all available
reporters.

Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Reviewed-by: Moshe Shemesh <moshe@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>