riscv: qemu: Split devicetree files for qemu_riscv32/64

This uses QEMU virt which creates its own devicetree.

Copy the existing empty version of this file, so splitting the existing
qemu-virt into two, since anyone actually trying to use this will need a
different devicetree for 32- and 64-bit machines.

Tested-by: Heinrich Schuchardt <heinrich.schuchardt@canaonical.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/qemu-riscv64_smode_defconfig b/configs/qemu-riscv64_smode_defconfig
index f575c12..6b77714 100644
--- a/configs/qemu-riscv64_smode_defconfig
+++ b/configs/qemu-riscv64_smode_defconfig
@@ -2,6 +2,7 @@
 CONFIG_SYS_MALLOC_LEN=0x800000
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_SIZE=0x20000
+CONFIG_DEFAULT_DEVICE_TREE="qemu-virt64"
 CONFIG_TARGET_QEMU_VIRT=y
 CONFIG_ARCH_RV64I=y
 CONFIG_RISCV_SMODE=y