dt: mdio-mux: Add mdio multiplexer driver node

Add integrated MDIO multiplexer driver node which contains
two mux PCIe bus and one ethernet bus along with phys
lying on these bus.

Signed-off-by: Pramod Kumar <pramod.kumar@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>