KVM: arm64: Expose host stage-2 manipulation helpers

We will need to manipulate the host stage-2 page-table from outside
mem_protect.c soon. Introduce two functions allowing this, and make
them usable to users of mem_protect.h.

Signed-off-by: Quentin Perret <qperret@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210809152448.1810400-14-qperret@google.com