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>