checks: Add Warning for stricter node name character checking

While '#', '?', '.', '+', '*', and '_' are considered valid characters,
their use is discouraged in recommended practices.

Testing this found a few cases of '.'. The majority of the warnings were
all from underscores.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>