net/mlx5e: Convert stats groups array to array of group pointers

Convert stats groups array to array of "stats group" pointers to allow
sharing and individual selection of groups per profile as illustrated in
the next patches.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>