configs: ast2600: enable DM_MDIO and MDIO driver

Enable DM_MDIO and Aspeed MDIO driver for AST2600 EVB.

Signed-off-by: Dylan Hung <dylan_hung@aspeedtech.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
diff --git a/configs/evb-ast2600_defconfig b/configs/evb-ast2600_defconfig
index ebdc06b..172b08e 100644
--- a/configs/evb-ast2600_defconfig
+++ b/configs/evb-ast2600_defconfig
@@ -64,7 +64,9 @@
 CONFIG_MMC_SDHCI_ASPEED=y
 CONFIG_PHY_REALTEK=y
 CONFIG_DM_ETH=y
+CONFIG_DM_MDIO=y
 CONFIG_FTGMAC100=y
+CONFIG_ASPEED_MDIO=y
 CONFIG_PHY=y
 CONFIG_PINCTRL=y
 CONFIG_RAM=y