airoha: Add eMMC config to defconfig

Enable Mediatek MMC driver in Airoha AN7581 EVB defconfig to add support
for it in default images.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
diff --git a/configs/an7581_evb_defconfig b/configs/an7581_evb_defconfig
index f5bd647..fa41a4b 100644
--- a/configs/an7581_evb_defconfig
+++ b/configs/an7581_evb_defconfig
@@ -77,3 +77,4 @@
 CONFIG_DM_SPI=y
 CONFIG_SHA512=y
 CONFIG_AIROHA_ETH=y
+CONFIG_MMC_MTK=y