x86: Adjust link device tree include file

This is currently set to coreboot.dtsi, but we cannot support this on
old device tree compilers (dtc <= 1.3), so adjust to use ARCH_CPU_DTS
to let the Makefile preprocessor sort this out.

Signed-off-by: Simon Glass <sjg@chromium.org>

board/chromebook-x86/dts/link.dts

@@ -1,6 +1,6 @@
 /dts-v1/;
 
-/include/ "coreboot.dtsi"
+/include/ ARCH_CPU_DTS
 
 / {
         #address-cells = <1>;