rockchip: enable fdt overlays for ROCK Pi 4 series

add CONFIG_OF_LIBFDT_OVERLAY=y to support fdt overlays.

Signed-off-by: FUKAUMI Naoki <naoki@radxa.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/configs/rock-pi-4-rk3399_defconfig b/configs/rock-pi-4-rk3399_defconfig
index 83721ce..8232c6f 100644
--- a/configs/rock-pi-4-rk3399_defconfig
+++ b/configs/rock-pi-4-rk3399_defconfig
@@ -84,3 +84,4 @@
 CONFIG_DISPLAY_ROCKCHIP_HDMI=y
 CONFIG_SPL_TINY_MEMSET=y
 CONFIG_ERRNO_STR=y
+CONFIG_OF_LIBFDT_OVERLAY=y