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/arch/arm/dts/Makefile b/arch/arm/dts/Makefile
index f634598..9130211 100644
--- a/arch/arm/dts/Makefile
+++ b/arch/arm/dts/Makefile
@@ -1142,6 +1142,9 @@
 
 dtb-$(CONFIG_XEN) += xenguest-arm64.dtb
 
+dtb-$(CONFIG_ARCH_OCTEONTX) += octeontx.dtb
+dtb-$(CONFIG_ARCH_OCTEONTX2) += octeontx.dtb
+
 dtb-$(CONFIG_TARGET_GE_BX50V3) += \
 	imx6q-bx50v3.dtb \
 	imx6q-b850v3.dtb \