warp7_bl33: configs: Enable CONFIG_OF_LIBFDT

In order to switch on DTB overlay support in WaRP7 BL33 we first need to
switch on LIBFDT support. Do that now.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
diff --git a/configs/warp7_bl33_defconfig b/configs/warp7_bl33_defconfig
index 29c4c24..d2cf84b 100644
--- a/configs/warp7_bl33_defconfig
+++ b/configs/warp7_bl33_defconfig
@@ -60,3 +60,4 @@
 CONFIG_FIT=y
 CONFIG_FIT_SIGNATURE=y
 CONFIG_FIT_VERBOSE=y
+CONFIG_OF_LIBFDT=y