ARM: dts: mdm9630: add support for pwr_event_irq

The pwr_event_irq is an asynchronous event routed to the
QGIC for waking up from low XO shutdown, even when the core
clocks are disabled. This interrupt is required for runtime
suspend/resume flows.

Change-Id: I8eed493f1c93f02dff2575380e8ce02c11b48983
Signed-off-by: Dov Levenglick <dovl@codeaurora.org>