Makefile: silence 'make clean'

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Wolfgang Denk <wd@denx.de>

Makefile

@@ -811,7 +811,7 @@ clean:
 	@rm -f $(obj)include/generated/asm-offsets.h
 	@rm -f $(obj)$(CPUDIR)/$(SOC)/asm-offsets.s
 	@rm -f $(TIMESTAMP_FILE) $(VERSION_FILE)
-	@$(MAKE) -C doc/DocBook/ cleandocs
+	@$(MAKE) -s -C doc/DocBook/ cleandocs
 	@find $(OBJTREE) -type f \
 		\( -name 'core' -o -name '*.bak' -o -name '*~' -o -name '*.su' \
 		-o -name '*.o'	-o -name '*.a' -o -name '*.exe'	\) -print \