[ARM] 3728/1: Restore missing CPU Hotplug irq helper

Patch from Thomas Gleixner

From: Thomas Gleixner <tglx@linutronix.de>

The genirq conversion of ARM lost a CPU Hotplug helper function.
Restore it.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>