bus: ti-sysc: Just use SET_NOIRQ_SYSTEM_SLEEP_PM_OPS

As Grygorii Strashko pointed out, the runtime PM use count of the
children can be whatever at suspend and we should not use it. So
let's just suspend ti-sysc at noirq level and get rid of some code.

Let's also remove the PM_SLEEP ifdef and use __maybe_unused as the
PM code already deals with the ifdefs.

Suggested-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>