riscv: ae350: Switch to use binman to generate u-boot.itb

Use the new BINMAN_STANDALONE_FDT option for AE350 based SPL defconfigs,
so that binman is now used to generate u-boot.itb.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Rick Chen <rick@andestech.com>
diff --git a/board/AndesTech/ax25-ae350/Kconfig b/board/AndesTech/ax25-ae350/Kconfig
index 321dd0c..e50f505 100644
--- a/board/AndesTech/ax25-ae350/Kconfig
+++ b/board/AndesTech/ax25-ae350/Kconfig
@@ -31,6 +31,7 @@
 	def_bool y
 	select RISCV_NDS
 	select SUPPORT_SPL
+	select BINMAN if SPL
 	imply SMP
 	imply SPL_RAM_SUPPORT
 	imply SPL_RAM_DEVICE