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/Makefile b/dts/Makefile
index 3437e54..d6c2c9d 100644
--- a/dts/Makefile
+++ b/dts/Makefile
@@ -10,10 +10,20 @@
 DEVICE_TREE := unset
 endif
 
+ifeq ($(CONFIG_OF_UPSTREAM),y)
+ifeq ($(CONFIG_ARM64),y)
+dt_dir := dts/upstream/src/arm64
+else
+dt_dir := dts/upstream/src/$(ARCH)
+endif
+else
+dt_dir := arch/$(ARCH)/dts
+endif
+
 ifneq ($(EXT_DTB),)
 DTB := $(EXT_DTB)
 else
-DTB := arch/$(ARCH)/dts/$(DEVICE_TREE).dtb
+DTB := $(dt_dir)/$(DEVICE_TREE).dtb
 endif
 
 $(obj)/dt-$(SPL_NAME).dtb: dts/dt.dtb $(objtree)/tools/fdtgrep FORCE
@@ -41,7 +51,7 @@
 
 PHONY += arch-dtbs
 arch-dtbs:
-	$(Q)$(MAKE) $(build)=arch/$(ARCH)/dts dtbs
+	$(Q)$(MAKE) $(build)=$(dt_dir) dtbs
 
 ifeq ($(CONFIG_SPL_BUILD),y)
 obj-$(CONFIG_OF_EMBED) := dt-spl.dtb.o
@@ -65,4 +75,5 @@
 # Let clean descend into dts directories
 subdir- += ../arch/arc/dts ../arch/arm/dts ../arch/m68k/dts ../arch/microblaze/dts	\
 	   ../arch/mips/dts ../arch/nios2/dts ../arch/powerpc/dts ../arch/riscv/dts	\
-	   ../arch/sandbox/dts ../arch/sh/dts ../arch/x86/dts ../arch/xtensa/dts
+	   ../arch/sandbox/dts ../arch/sh/dts ../arch/x86/dts ../arch/xtensa/dts	\
+	   ./upstream/src/arm64 ./upstream/src/$(ARCH)