irqchip/gic-v3-its: Track LPI distribution on a per CPU basis

In order to improve the distribution of LPIs among CPUs, let start by
tracking the number of LPIs assigned to CPUs, both for managed and
non-managed interrupts (as separate counters).

Signed-off-by: Marc Zyngier <maz@kernel.org>
Tested-by: John Garry <john.garry@huawei.com>
Link: https://lore.kernel.org/r/20200515165752.121296-2-maz@kernel.org