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

Move the LPI property table allocation into its own function, as
this is going to be required for those associated with VMs in
the future.

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