dt-bindings: net: sparx5: Add sparx5-switch bindings

Document the Sparx5 switch device driver bindings

Signed-off-by: Steen Hegelund <steen.hegelund@microchip.com>
Signed-off-by: Lars Povlsen <lars.povlsen@microchip.com>
Signed-off-by: Bjarni Jonasson <bjarni.jonasson@microchip.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>