mlxsw: spectrum_router: Add basic ipv4 router initialization

Create a skeleton router file and do basic HW initialization of router.

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