pm: Use stats subsys for tracking system states

Uses the stats subsys to provide simple but useful debugging stats for
power management state changes and timing.

Removes the no longer needed PM_DEBUG config option

Replaces the use of PM_DEBUG for a test clock output pin for mec1501 and
adds in its place an SoC Kconfig option to enable it.

Adds a STATS_SET macro for assigning a value to a stat group field

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>