dts/upsteam: Add Makefile for xtensa

It is required to get it xtensa OF_UPSTREAM work.

Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Tested-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>