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/arch/arm/dts/Makefile b/arch/arm/dts/Makefile
index eaad3b6..2d92b2f 100644
--- a/arch/arm/dts/Makefile
+++ b/arch/arm/dts/Makefile
@@ -1162,7 +1162,7 @@
 
 dtb-$(CONFIG_TARGET_EA_LPC3250DEVKITV2) += lpc3250-ea3250.dtb
 
-dtb-$(CONFIG_ARCH_QEMU) += qemu-arm.dtb
+dtb-$(CONFIG_ARCH_QEMU) += qemu-arm.dtb qemu-arm64.dtb
 
 targets += $(dtb-y)