net: phy: add amlogic g12a mdio mux support

Add support for the mdio mux and internal phy glue of the g12a SoC family

Reviewed-by: Neil Armstrong <narmstrong@baylibre.com> # clk parts
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>