kbuild: disable dtc simple_bus_reg warnings by default

The updated version of dtc has a bug fix for simple_bus_reg warnings
and lots of warnings are generated now. So disable this warning by
default.
Signed-off-by: Rob Herring <robh@kernel.org>

scripts/Makefile.lib

@@ -252,6 +252,7 @@ DTC_FLAGS += -Wno-unit_address_vs_reg \
 	-Wno-alias_paths \
 	-Wno-graph_child_address \
 	-Wno-graph_port \
+	-Wno-simple_bus_reg \
 	-Wno-unique_unit_address \
 	-Wno-pci_device_reg
 endif