PM / OPP: Add binding for 'opp-suspend'

On few platforms, for power efficiency, we want the device to be
configured for a specific OPP while we put the device in suspend state.

Add an optional property in operating-points-v2 bindings for that.

Suggested-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Nishanth Menon <nm@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>