irqchip/gic-v3-its: Allow use of LPI tables in reserved memory

If the LPI tables have been reserved with the EFI reservation
mechanism, we assume that these tables are safe to use even
when we find the redistributors to have LPIs enabled at
boot time, meaning that kexec can now work with GICv3.

You're welcome.

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>