PM / sleep: Enhance test_suspend option with repeat capability

Enhanced test_suspend boot paramter to repeat tests multiple times,
by adding optional repeat count. The new boot param syntax:
test_suspend="mem|freeze|standby[,N]"

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>