KVM: arm64: Introduce kvm_counter_compute_delta() helper

Refactor kvm_timer_compute_delta() and extract a helper that
compute the delta (in ns) between a given timer and an arbitrary
value.

No functional change expected.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220419182755.601427-5-maz@kernel.org