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/configs/qemu-riscv64_spl_defconfig b/configs/qemu-riscv64_spl_defconfig
index 897adf6..6c68048 100644
--- a/configs/qemu-riscv64_spl_defconfig
+++ b/configs/qemu-riscv64_spl_defconfig
@@ -2,12 +2,14 @@
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_SIZE=0x20000
 CONFIG_SPL=y
+CONFIG_DEFAULT_DEVICE_TREE="qemu-virt"
 CONFIG_TARGET_QEMU_VIRT=y
 CONFIG_ARCH_RV64I=y
 CONFIG_RISCV_SMODE=y
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_FIT=y
 CONFIG_SPL_LOAD_FIT_ADDRESS=0x80200000
+# CONFIG_USE_SPL_FIT_GENERATOR is not set
 CONFIG_DISPLAY_CPUINFO=y
 CONFIG_DISPLAY_BOARDINFO=y
 # CONFIG_CMD_MII is not set