dt-bindings: net: add amlogic gxl mdio multiplexer

Add documentation for the MDIO bus multiplexer found on the Amlogic GXL
SoC family

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>