dts, arm: Remove $(MACHINE) variable from dtbs make recipes

The machine description is not needed to build dtb files.
Signed-off-by: Robert Richter <rrichter@cavium.com>

arch/arm/Makefile

@@ -314,10 +314,10 @@ $(INSTALL_TARGETS):
 PHONY += dtbs dtbs_install
 
 dtbs: prepare scripts
-	$(Q)$(MAKE) $(build)=$(boot)/dts MACHINE=$(MACHINE)
+	$(Q)$(MAKE) $(build)=$(boot)/dts
 
 dtbs_install:
-	$(Q)$(MAKE) $(dtbinst)=$(boot)/dts MACHINE=$(MACHINE)
+	$(Q)$(MAKE) $(dtbinst)=$(boot)/dts
 
 # We use MRPROPER_FILES and CLEAN_FILES now
 archclean: