PM / wakeup: export pm_system_wakeup symbol

Export pm_system_wakeup function to allow irq handlers to deal with system
wakeup.

This is needed for shared IRQ lines where one of the handler is registered
with IRQF_NO_SUSPEND, while the other ones want to configure it as a wakeup
source.

In this specific case, irq core does not handle the wakeup process and
leave the decision to each irq handler.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>