binding: Make "mdio-parent-bus" property from mandatory to optional

Change "mdio-parent-bus" from mandatory section to optional
as it won't be required by integrated MDIO multiplexer
which has bus selection and mdio transaction generation logic,
integrated inside.

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