kbuild: Silence unavoidable dtc overlay warnings

Much effort has been put into finding ways to avoid warnings from dtc
about overlays, usually to do with the presence of #address-cells and
size-cells, but not exclusively so. Since the issues being warned about
are harmless, suppress the warnings to declutter the build output and
to avoid alarming users.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>

scripts/Makefile.lib

@@ -348,6 +348,12 @@ cmd_dtco = mkdir -p $(dir ${dtc-tmp}) ; \
 	$(DTC) -@ -H epapr -O dtb -o $@ -b 0 \
 		-i $(dir $<) $(DTC_FLAGS) \
 		-Wno-interrupts_property \
+		-Wno-label_is_string \
+		-Wno-reg_format \
+		-Wno-pci_device_bus_num \
+		-Wno-i2c_bus_reg \
+		-Wno-spi_bus_reg \
+		-Wno-avoid_default_addr_size \
 		-d $(depfile).dtc.tmp $(dtc-tmp) ; \
 	cat $(depfile).pre.tmp $(depfile).dtc.tmp > $(depfile)