dts, arm/arm64: Remove dtbs build rules in sub-makes

Add dtb files to build targets and let kbuild handle them. Thus,
special dtbs rules can be removed. This eases Makefiles and the
implementation of the support of vendor dtb subdirectories.

Signed-off-by: Robert Richter <rrichter@cavium.com>
diff --git a/arch/arm/boot/dts/Makefile b/arch/arm/boot/dts/Makefile
index e0d348b..6e784fa 100644
--- a/arch/arm/boot/dts/Makefile
+++ b/arch/arm/boot/dts/Makefile
@@ -517,13 +517,7 @@
 	dove-dove-db.dtb
 dtb-$(CONFIG_ARCH_MEDIATEK) += mt6589-aquaris5.dtb
 
-targets += dtbs dtbs_install
-targets += $(dtb-y)
 endif
 
-# *.dtb used to be generated in the directory above. Clean out the
-# old build results so people don't accidentally use them.
-dtbs: $(addprefix $(obj)/, $(dtb-y))
-	$(Q)rm -f $(obj)/../*.dtb
-
-clean-files := *.dtb
+always		:= $(dtb-y)
+clean-files	:= *.dtb