imx8m{m,n}_venice_defconfig: add DT overlay support

enable CONFIG_OF_LIBFDT_OVERLAY to support applying dt overlays in
U-Boot.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
diff --git a/configs/imx8mn_venice_defconfig b/configs/imx8mn_venice_defconfig
index 2f5ea62..7628667 100644
--- a/configs/imx8mn_venice_defconfig
+++ b/configs/imx8mn_venice_defconfig
@@ -137,3 +137,4 @@
 CONFIG_USB_GADGET_DOWNLOAD=y
 CONFIG_IMX_WATCHDOG=y
 CONFIG_HEXDUMP=y
+CONFIG_OF_LIBFDT_OVERLAY=y