RDMA/counter: Add a descriptor in struct rdma_hw_stats

Add a counter statistic descriptor structure in rdma_hw_stats. In addition
to the counter name, more meta-information will be added.  This code
extension is needed for optional-counter support in the following patches.

Link: https://lore.kernel.org/r/20211008122439.166063-4-markzhang@nvidia.com
Signed-off-by: Aharon Landau <aharonl@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Mark Zhang <markzhang@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>