net/mlx5: Move debugfs entries to separate struct

Move the debugfs entry pointers under priv to their own struct.
Add get function for device debugfs root.

Signed-off-by: Moshe Shemesh <moshe@nvidia.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>