kbuild: Enable dtc 'pci_device_reg' warning by default

There's only a couple of instances of the 'pci_device_reg' warnings left
and they look legit, so let's enable the warning by default.

Cc: Heiko Stuebner <heiko@sntech.de>
Cc: Nicolas Saenz Julienne <nsaenz@kernel.org>
Cc: soc@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210820165011.3257112-1-robh@kernel.org/