dts, kbuild: Factor out dtbs install rules to Makefile.dtbinst

Move dtbs install rules to Makefile.dtbinst. This change is needed to
implement support for dts vendor subdirs. The change makes Makefiles
easier and smaller as no longer the dtbs_install rule needs to be
defined. Another advantage is that install goals are not encoded in
targets anymore (%.dtb_dtbinst_).

Signed-off-by: Robert Richter <rrichter@cavium.com>
diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index a963b3a..8e9167a 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -71,9 +71,13 @@
 	$(Q)$(MAKE) $(build)=$(boot)/dts $(boot)/dts/$@
 
 PHONY += dtbs dtbs_install
-dtbs dtbs_install: prepare scripts
+
+dtbs: prepare scripts
 	$(Q)$(MAKE) $(build)=$(boot)/dts $@
 
+dtbs_install:
+	$(Q)$(MAKE) $(dtbinst)=$(boot)/dts
+
 PHONY += vdso_install
 vdso_install:
 	$(Q)$(MAKE) $(build)=arch/arm64/kernel/vdso $@