irqchip/gic-v3-its: Split out pending table allocation

Just as for the property table, let's move the pending table
allocation to a separate function.

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