PM / OPP: Don't create debugfs "supply-0" directory unnecessarily

We create "supply-0" debugfs directory even if the device doesn't do
voltage scaling. That looks confusing, as if the regulator is found but
we never managed to get voltage levels for it.

Avoid creating such a directory unnecessarily.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>