configs: update Amlogic Meson G12A, G12B & SM1 configs for MDIO MUX

Use DM_MDIO and the new G12A MDIO MUX driver for G12A, G12B & SM1 boards.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
diff --git a/configs/khadas-vim3l_defconfig b/configs/khadas-vim3l_defconfig
index c187792..8cc0214 100644
--- a/configs/khadas-vim3l_defconfig
+++ b/configs/khadas-vim3l_defconfig
@@ -41,6 +41,9 @@
 CONFIG_DM_MTD=y
 CONFIG_DM_SPI_FLASH=y
 CONFIG_SPI_FLASH_WINBOND=y
+CONFIG_DM_MDIO=y
+CONFIG_DM_MDIO_MUX=y
+CONFIG_MDIO_MUX_MESON_G12A=y
 CONFIG_PHY_REALTEK=y
 CONFIG_DM_ETH=y
 CONFIG_ETH_DESIGNWARE=y