arm64: use new common dtc rule

The current rules have the .dtb files build in a different directory
from the .dts files. This patch changes arm64 to use the generic dtb
rule which builds .dtb files in the same directory as the source .dts.

This requires moving parts of arch/arm64/boot/Makefile into newly created
arch/arm64/boot/dts/Makefile, and updating arch/arm64/Makefile to call the
new Makefile.

Acked-by: Catalin Marinas <[e-mail]>
Cc: [e-mail]
Signed-off-by: Stephen Warren <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

arch/arm64/Makefile

···
 	$(Q)$(MAKE) $(build)=$(boot) MACHINE=$(MACHINE) $@
 
 %.dtb:
-	$(Q)$(MAKE) $(build)=$(boot) MACHINE=$(MACHINE) $(boot)/$@
+	$(Q)$(MAKE) $(build)=$(boot)/dts MACHINE=$(MACHINE) $(boot)/dts/$@
 
 # We use MRPROPER_FILES and CLEAN_FILES now
 archclean:

arch/arm64/boot/Makefile

···
 $(obj)/Image.gz: $(obj)/Image FORCE
 	$(call if_changed,gzip)
 
-$(obj)/%.dtb: $(src)/dts/%.dts
-	$(call cmd,dtc)
-
 install: $(obj)/Image
 	$(CONFIG_SHELL) $(srctree)/$(src)/install.sh $(KERNELRELEASE) \
 	$(obj)/Image System.map "$(INSTALL_PATH)"
···
 zinstall: $(obj)/Image.gz
 	$(CONFIG_SHELL) $(srctree)/$(src)/install.sh $(KERNELRELEASE) \
 	$(obj)/Image.gz System.map "$(INSTALL_PATH)"
-
-clean-files += *.dtb

arch/arm64/boot/dts/Makefile

···
+targets += dtbs
+
+dtbs: $(addprefix $(obj)/, $(dtb-y))
+
+clean-files := *.dtb