mlxsw: spectrum_switchdev: Add support for router port in SMID entries

In Spectrum, MDB entries point to MID entries, that indicate which ports a
packet should be forwarded to. Add the support in creating MID entries that
forward the packet to the Spectrum router port.

This will be later used to handle the bridge mrouter port switchdev
notifications.

Signed-off-by: Yotam Gigi <yotamg@mellanox.com>
Reviewed-by: Nogah Frankel <nogahf@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>