dt-bindings: net: Convert MDIO mux bindings to DT schema

Convert the common MDIO mux bindings to DT schema.

Drop the example from mdio-mux.yaml as mdio-mux-gpio.yaml has the same one.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Ray Jui <rjui@broadcom.com>
Cc: Scott Branden <sbranden@broadcom.com>
Cc: bcm-kernel-feedback-list@broadcom.com
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Heiner Kallweit <hkallweit1@gmail.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: netdev@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20210526181411.2888516-1-robh@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>