irqchip: brcmstb-l2: Define an irq_pm_shutdown function

The Broadcom STB platforms support S5 and we allow specific hardware
wake-up events to take us out of this state. Because we were not
defining an irq_pm_shutdown() function pointer, we would not be
correctly masking non-wakeup events, which would result in spurious
wake-ups from sources that were not explicitly configured for wake-up.

Fixes: 7f646e92 ("irqchip: brcmstb-l2: Add Broadcom Set Top Box Level-2 interrupt controller")
Acked-by: Gregory Fong <gregory.0xf0@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>