net/mlx5e: Add CONFIG_MLX5_EN_ARFS for accelerated flow steering support

Add new mlx5 Kconfig flag to allow selecting accelerated flow steering
support, and compile out en_arfs.c if not selected.

Move arfs declarations and definitions to en/fs.h header file.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Moshe Shemesh <moshe@mellanox.com>