irqchip/gic-v3-its: Use full range of LPIs

As we used to represent the LPI range using a bitmap, we were reducing
the number of LPIs to at most 64k in order to preserve memory.

With our new allocator, there is no such need, as dealing with 2^16
or 2^32 LPIs takes the same amount of memory.

So let's use the number of IntID bits reported by the GIC instead of
an arbitrary limit.

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