drivers: soc: rpm_stats: Add sysfs support for RPM stats

Allow reading RPM stats information using sysfs nodes. Debugfs is not
available on SE linux.

The stats are available at
	/sys/power/system_sleep/stats

Change-Id: I73962727ff410094ffd57a3ac0869819d0663b37
Signed-off-by: Lina Iyer <ilina@codeaurora.org>