binding: mdio-mux: Add DT binding doc for Broadcom MDIO bus multiplexer

Add DT binding doc for Broadcom MDIO bus multiplexer driver.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Pramod Kumar <pramod.kumar@broadcom.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>