dt-bindings: opp: Allow multi-worded OPP entry name

Not all OPP entries fit into a single word. In particular NVIDIA Tegra OPP
tables use multi-word names. Allow OPP entry to have multi-worded name
separated by hyphen. This silences DT checker warnings about wrong naming
scheme.

Reviewed-by: David Heidelberg <david@ixit.cz>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>