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

By utilizing the newly introduced BINMAN_STANDALONE_FDT option, along
with a new dedicated device tree source file for the QEMU virt target
used for binman only, we can now use binman to generate u-boot.itb.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/riscv/dts/qemu-virt.dts b/arch/riscv/dts/qemu-virt.dts
new file mode 100644
index 0000000..fecff54
--- /dev/null
+++ b/arch/riscv/dts/qemu-virt.dts
@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright (C) 2021, Bin Meng <bmeng.cn@gmail.com>
+ */
+
+/dts-v1/;
+
+#include "binman.dtsi"