ARM: meson-gx: Reset GXL/GXM to external PHY when not using internal PHY

When using External PHY, reset the mux to use the external PHY in case U-Boot
was chainloaded from a misconfigured bootloader.

Fixes: 33e33780 ("ARM: meson: rework soc arch file to prepare for new SoC")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>