arm64: dts: uniphier: fix W=2 build warnings

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>