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/Makefile b/arch/riscv/dts/Makefile
index 3780334..26ef853 100644
--- a/arch/riscv/dts/Makefile
+++ b/arch/riscv/dts/Makefile
@@ -2,6 +2,7 @@
 
 dtb-$(CONFIG_TARGET_AX25_AE350) += ae350_32.dtb ae350_64.dtb
 dtb-$(CONFIG_TARGET_MICROCHIP_ICICLE) += microchip-mpfs-icicle-kit.dtb
+dtb-$(CONFIG_TARGET_QEMU_VIRT) += qemu-virt.dtb
 dtb-$(CONFIG_TARGET_SIFIVE_UNLEASHED) += hifive-unleashed-a00.dtb
 dtb-$(CONFIG_TARGET_SIPEED_MAIX) += k210-maix-bit.dtb