devlink: add parameter documentation for the mlx4 driver

The mlx5 and mlxsw drivers have driver-specific documentation for the
devlink features they support. No such file was added for mlx4.

Add a file to document the mlx4 devlink support. Initially it contains
only the devlink parameters.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Cc: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>