PM / OPP: Parse all power-supply related bindings together

Move all DT parsing for the power supplies to a single function, rather
than keeping them at separate places. This will help manage things
properly.

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>