KVM: arm64: vgic-its: Spin off helper for finding ITS by doorbell addr

The fast path will soon need to find an ITS by doorbell address, as the
translation caches will become local to an ITS. Spin off a helper to do
just that.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240422200158.2606761-10-oliver.upton@linux.dev


Signed-off-by: Marc Zyngier <maz@kernel.org>