KVM: arm64: Introduce helper to retrieve a PTE and its level

It is becoming a common need to fetch the PTE for a given address
together with its level. Add such a helper.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Quentin Perret <qperret@google.com>
Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Link: https://lore.kernel.org/r/20210726153552.1535838-2-maz@kernel.org