irqchip/gic-v3-its: Make LPI allocation optional on device creation

The normal course of action when allocating the ITS' view of a
device is to allocate the corresponding LPIs. But we're about
to introduce devices that borrow their interrupts from
some other entities.

So let's make the allocation optional.

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