net: Add Amlogic GXL MDIO Mux driver

Port the mdio-mux-meson-gxl.c Linux driver introduced in [1],
and adapt it to U-Boot.

This driver is needed to boot U-Boot with Linux DT since v6.4,
since it switched the MDIO mux from the mmio to a proper GXL driver.

[1] 9a24e1ff4326 ("net: mdio: add amlogic gxl mdio mux support")

Link: https://lore.kernel.org/r/20231213-u-boot-gxl-mdio-mux-v2-1-c56bb02a75ea@linaro.org
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>