arm: octeontx: Add an empty devicetree file

Add an empty file to prevent build errors when building with
CONFIG_OF_SEPARATE enabled.

Unfortunately there are no build instructions in the U-Boot tree to enable
a real file to be created.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/octeontx_81xx_defconfig b/configs/octeontx_81xx_defconfig
index d0728ac..d871be8 100644
--- a/configs/octeontx_81xx_defconfig
+++ b/configs/octeontx_81xx_defconfig
@@ -12,6 +12,7 @@
 CONFIG_ENV_SECT_SIZE=0x10000
 CONFIG_TARGET_OCTEONTX_81XX=y
 CONFIG_DM_GPIO=y
+CONFIG_DEFAULT_DEVICE_TREE="octeontx"
 CONFIG_DEBUG_UART_BASE=0x87e028000000
 CONFIG_DEBUG_UART_CLOCK=24000000
 CONFIG_DEBUG_UART=y