dt-bindings: net: Add a YAML schemas for the generic MDIO options

The MDIO buses have a number of available device tree properties that can
be used in their device tree node. Add a YAML schemas for those.

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>