KVM: arm64: Add support for stage-2 page-aging in generic page-table

Add stage-2 mkyoung(), mkold() and is_young() operations to the generic
page-table code.

Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Quentin Perret <qperret@google.com>
Link: https://lore.kernel.org/r/20200911132529.19844-11-will@kernel.org