irqchip/gic-v3-its: Keep track of property table's PA and VA

We're currently only tracking the page allocated to contain the
property table by its struct page. In the future, it is going to
be convenient to track both PA and VA for that page instead. Let's
do that.

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>