kbuild: update DTC warning settings for bus and node/property name checks

Recent commits of DTC introduced new warnings checking PCI and simple
buses, unit address formatting, and stricter node and property name
checking.  Disable the new DTC warnings by default.  As before,
warnings are enabled with W=*.  The strict node and property name
checks are a bit subjective, so they are only enabled for W=2.
(This policy reflects the commit 8654cb8d0371 of Linux.)

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>