irqchip/gic-v3-its: Reduce minimum LPI allocation to 1 for PCI devices

Allocating a minimum of 32 LPIs per PCI device, let's reduce it to
be just 1, as most devices do not need that many interrupts.

We still have to special-case DevID 0, as there is plenty of broken
HW around where the PCI RID is not presented as a DevID to the ITS,
and all the devices are presented as DevID 0. In this case, we keep
the 32 minimal allocation.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>