checks: Add Warning for stricter property name character checking

While '?', '.', '+', '*', and '_' are considered valid characters their
use is discouraged in recommended practices. '#' is also only
recommended to be used at the beginning of property names.

Testing this found one typo error with '.' used instead of ','. The
rest 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>