irqchip/gic-v3-its: Rework LPI freeing

Rework LPI deallocation so that it can be reused by the v4 support
code.

Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>