Allow u-boot to build DTB from a different directory tree such that
*-u-boot.dtsi files can be included from a common location. Currently
that location is arch/$(ARCH)/dts/, so statically define that common
location.

This is needed for platform owners to start building DTB files from
devicetree-rebasing directory but still being able to include
*-u-boot.dtsi files.

Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>