dt-bindings: net: davinci-mdio: convert bindings to json-schema

Now that we have the DT validation in place, let's convert the device tree
bindings for the TI SoC Davinci/OMAP/Keystone2 MDIO Controllerr over to a
YAML schemas.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Rob Herring <robh@kernel.org>