configs: update Amlogic Meson GXL & GXM config for MDIO MUX

Use DM_MDIO and the new MMIOREG MDIO MUX driver for GXL & GXM board.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
diff --git a/configs/libretech-s912-pc_defconfig b/configs/libretech-s912-pc_defconfig
index a515833..c54304a 100644
--- a/configs/libretech-s912-pc_defconfig
+++ b/configs/libretech-s912-pc_defconfig
@@ -37,6 +37,9 @@
 CONFIG_MTD=y
 CONFIG_DM_SPI_FLASH=y
 CONFIG_SPI_FLASH_GIGADEVICE=y
+CONFIG_DM_MDIO=y
+CONFIG_DM_MDIO_MUX=y
+CONFIG_MDIO_MUX_MMIOREG=y
 CONFIG_PHY_REALTEK=y
 CONFIG_DM_ETH=y
 CONFIG_ETH_DESIGNWARE=y