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Fix warnings like follows:

Warning (node_name_chars_strict): Character '_' not recommended in ...

Commit 8654cb8d

 ("dtc: update warning settings for new bus and
node/property name checks") says these checks are a bit subjective,
but Rob also says to not add new W=2 warnings.

The exising warnings should be fixed in order to catch new ones
easily.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>