Documentation: add devlink param file for mlxsw driver

Add initial documentation file for devlink params of mlxsw driver. Only
"fw_load_policy" is now supported.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>