IB/mlx5: Add counters read support

This patch implements the uverbs counters read API, it will use the
specific read counters function to the given type to accomplish its
task.

Reviewed-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Raed Salem <raeds@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>