irqchip/gic-v3-its: Split property table clearing from allocation

As we're going to reuse some pre-allocated memory for the property
table, split out the zeroing of that table into a separate function
for later use.

Tested-by: Jeremy Linton <jeremy.linton@arm.com>
Tested-by: Bhupesh Sharma <bhsharma@redhat.com>
Tested-by: Lei Zhang <zhang.lei@jp.fujitsu.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>