arm: qemu: Add a devicetree file for qemu_arm

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 7f622fe..eaad3b6 100644
--- a/arch/arm/dts/Makefile
+++ b/arch/arm/dts/Makefile
@@ -1162,6 +1162,8 @@
 
 dtb-$(CONFIG_TARGET_EA_LPC3250DEVKITV2) += lpc3250-ea3250.dtb
 
+dtb-$(CONFIG_ARCH_QEMU) += qemu-arm.dtb
+
 targets += $(dtb-y)
 
 # Add any required device tree compiler flags here