dt-bindings: net: convert mdio-mux-meson-g12a.txt to dt-schema

Convert MDIO bus multiplexer/glue of Amlogic G12a SoC family bindings
to dt-schema.

Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>