arm: qemu: Add a devicetree file for qemu_arm64 virt

This uses QEMU virt which creates its own devicetree.

Add an empty version of this file, so that we can at least build this
board when devicetrees are required.

Tested-by: Heinrich Schuchardt <heinrich.schuchardt@canaonical.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/qemu_arm64_defconfig b/configs/qemu_arm64_defconfig
index 0ac3b47..7d03eb7 100644
--- a/configs/qemu_arm64_defconfig
+++ b/configs/qemu_arm64_defconfig
@@ -5,6 +5,7 @@
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_SIZE=0x40000
 CONFIG_ENV_SECT_SIZE=0x40000
+CONFIG_DEFAULT_DEVICE_TREE="qemu-arm64"
 CONFIG_DEBUG_UART_BASE=0x9000000
 CONFIG_DEBUG_UART_CLOCK=0
 CONFIG_DEBUG_UART=y