dt-bindings: net: phy: add g12a mdio mux documentation

Add documentation for the device tree bindings of the MDIO mux of Amlogic
g12a SoC family

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>