arm/arm64: KVM: vgic: handle out-of-range MMIO accesses

Now that we can (almost) dynamically size the number of interrupts,
we're facing an interesting issue:

We have to evaluate at runtime whether or not an access hits a valid
register, based on the sizing of this particular instance of the
distributor. Furthermore, the GIC spec says that accessing a reserved
register is RAZ/WI.

For this, add a new field to our range structure, indicating the number
of bits a single interrupts uses. That allows us to find out whether or
not the access is in range.

Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>