clocksource: Add new feature flag CLOCK_SOURCE_SUSPEND_NONSTOP

Some x86 processors have a TSC clocksource, which continues to run
even when system is suspended. Also most OMAP platforms have a
32 KHz timer which has similar capability. Add a feature flag so that
it could be utilized.

Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>