net: add Amlogic Meson G12A MDIO MUX driver

The Amlogic G12A & compatible SoCs embeds a mux to either communicate with
the external PHY or the internal 10/100 PHY.

This adds support for this mux as a MDIO MUX device.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>