irqchip/gic-v3-its: Allow use of pre-programmed LPI tables

In order to cope with kexec and GICv3, let's try and spot when
we're booting with LPIs already enabled, and the tables already
programmed into the redistributors.

This code is currently guarded by a predicate that is always false,
meaning this is not functionnal just yet.

Reviewed-by: Julien Thierry <julien.thierry@arm.com>
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>