arm: bcm7xxx: Add a devicetree file

Add an empty devicetree file for these boards. It seems to be possible to
obtain a real one from another bootloader called 'bolt' but I will leave
this to the maintainer.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/bcm7260_defconfig b/configs/bcm7260_defconfig
index 257d810..2b527b6 100644
--- a/configs/bcm7260_defconfig
+++ b/configs/bcm7260_defconfig
@@ -7,6 +7,7 @@
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_SIZE=0x10000
 CONFIG_ENV_OFFSET=0x814800
+CONFIG_DEFAULT_DEVICE_TREE="bcm7xxx"
 CONFIG_ENV_OFFSET_REDUND=0x824800
 CONFIG_SYS_LOAD_ADDR=0x02000000
 CONFIG_FIT=y