IB/core: Introduce counters read verb

The user supplies counters instance and a reference to an output array of
uint64_t.  The driver reads the hardware counters values and writes them
to the output index location in the user supplied array.  All counters
values are represented as uint64_t types.

To be able to successfully read the data the counters must be first bound
to an IB object.

Downstream patches will present binding method for flow counters.

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>