RDMA/mlx5: Separate counters from main.c

RDMA/mlx5: Separate counters from main.c

There are number of counters types supported in mlx5_ib: HW counters,
congestion counters, Q-counters and flow counters. Almost all supporting
code was placed in main.c that made almost impossible to maintain the code
anymore. Let's create separate code namespace for the counters to easy
future generalization effort.

Link: https://lore.kernel.org/r/20200702081809.423482-4-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>