net: mdio: add amlogic gxl mdio mux support

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

Reported-by: Da Xue <da@lessconfused.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>