arm: qemu-ppce500: Add a devicetree file

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.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/qemu-ppce500_defconfig b/configs/qemu-ppce500_defconfig
index f386f52..ae4cd8b 100644
--- a/configs/qemu-ppce500_defconfig
+++ b/configs/qemu-ppce500_defconfig
@@ -1,6 +1,7 @@
 CONFIG_PPC=y
 CONFIG_SYS_TEXT_BASE=0xf01000
 CONFIG_ENV_SIZE=0x2000
+CONFIG_DEFAULT_DEVICE_TREE="qemu-ppce500"
 CONFIG_MPC85xx=y
 # CONFIG_CMD_ERRATA is not set
 CONFIG_TARGET_QEMU_PPCE500=y