dts: Add alternative location for upstream DTB builds

Allow platform owners to mirror devicetree files from devitree-rebasing
directory into dts/upstream/src/$(ARCH) (special case for arm64). Then
build then along with any *-u-boot.dtsi file present in arch/$(ARCH)/dts
directory. Also add a new Makefile for arm64.

This will help easy migration for platforms which currently are compliant
with upstream Linux kernel devicetree files.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
diff --git a/dts/Kconfig b/dts/Kconfig
index def0e17..b9b6367 100644
--- a/dts/Kconfig
+++ b/dts/Kconfig
@@ -85,6 +85,22 @@
 	  enables a live tree which is available after relocation,
 	  and can be adjusted as needed.
 
+config OF_UPSTREAM
+	bool "Enable use of devicetree imported from Linux kernel release"
+	help
+	  Traditionally, U-Boot platforms used to have their custom devicetree
+	  files or copy devicetree files from Linux kernel which are hard to
+	  maintain and can usually get out-of-sync from Linux kernel. This
+	  option enables platforms to migrate to devicetree-rebasing repo where
+	  a regular sync will be maintained every major Linux kernel release
+	  cycle. However, platforms can still have some custom u-boot specific
+	  bits maintained as part of *-u-boot.dtsi files.
+
+	  Note: This option should be set in Kconfig, for the SoC as a whole.
+	  However, newer boards whose devicetree source files haven't landed in
+	  the dts/upstream subtree, they can override this option to have the
+	  DT build from existing U-Boot tree location instead.
+
 choice
 	prompt "Provider of DTB for DT control"
 	depends on OF_CONTROL