PM / OPP: Manage supply's voltage/current in a separate structure

This is a preparatory step for multiple regulator per device support.
Move the voltage/current variables to a new structure.

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