dt-bindings: net: mdio: Allow any child node name

An MDIO bus can have devices other than ethernet PHYs on it, so it
should allow for any node name rather than just 'ethernet-phy'.

Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Heiner Kallweit <hkallweit1@gmail.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: netdev@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20211206174139.2296497-1-robh@kernel.org