PM / sleep: Make lock/unlock_system_sleep() available to kernel modules

Since pm_mutex is not exported using lock/unlock_system_sleep() from
inside a kernel module causes a "pm_mutex undefined" linker error.
Hence move lock/unlock_system_sleep() into kernel/power/main.c and
export these.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>