mlxsw: spectrum_router: Implement fib4 add/del switchdev obj ops

Implement ipv4 FIB entries addition and removal. Initially, we support
local and broadcast routes using "ip2me" trap action.
Also, unicast routes without nexthop are supported using "local" action.

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