PM / OPP: Add support for opp-suspend

With "operating-points-v2" bindings, it's possible to specify the OPP to
which the device must be switched, before suspending.

This patch adds support for getting that information.

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