KVM: arm64: Introduce KVM_PGTABLE_S2_IDMAP stage 2 flag

Introduce a new stage 2 configuration flag to specify that all mappings
in a given page-table will be identity-mapped, as will be the case for
the host. This allows to introduce sanity checks in the map path and to
avoid programming errors.

Suggested-by: Will Deacon <will@kernel.org>
Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210319100146.1149909-34-qperret@google.com