net/mlx5e: Add Ethernet driver debugfs

Similar to the mlx5_core debugfs, lay the groundwork for mlx5e debugfs
files under /sys/kernel/debug/mlx5/<pci>/nic/..

Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>