irqchip/gic: Prepare for more than 16 PPIs

GICv3.1 allows up to 80 PPIs (16 legaci PPIs and 64 Extended PPIs),
meaning we can't just leave the old 16 hardcoded everywhere.

We also need to add the infrastructure to discover the number of PPIs
on a per redistributor basis, although we still pretend there is only
16 of them for now.

No functional change.

Signed-off-by: Marc Zyngier <maz@kernel.org>