kbuild: disable dtc simple_bus_reg warnings by default

[ Upstream commit 70523a3c ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>