irqchip/gic-v3-its: Generalize device table allocation

As we want to use 2-level tables for VCPUs, let's hack the device
table allocator in order to make it slightly more generic. It
will get reused in subsequent patches.

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>