We cannot preempt this lock on -rt as we are in an
interrupt disabled region and about to go into deep sleep.

In mainline this change documents the low level nature of
the lock - otherwise there's no functional difference. Lockdep
and Sparse checking will work as usual.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Cc: Len Brown <len.brown@intel.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>