arc: use common make variables for dtb builds

Use dtb-y and always make variables to build dtbs instead of explicit
dtbs rule. This is in preparation to support building all dtbs.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Vineet Gupta <vgupta@synopsys.com>
diff --git a/arch/arc/Makefile b/arch/arc/Makefile
index 8a27a48..cf0cf34 100644
--- a/arch/arc/Makefile
+++ b/arch/arc/Makefile
@@ -121,7 +121,7 @@
 	$(Q)$(MAKE) $(build)=$(boot)/dts $(boot)/dts/$@
 
 dtbs: scripts
-	$(Q)$(MAKE) $(build)=$(boot)/dts dtbs
+	$(Q)$(MAKE) $(build)=$(boot)/dts
 
 archclean:
 	$(Q)$(MAKE) $(clean)=$(boot)