KVM: arm64: Hook up ->page_count() for hypervisor stage-1 page-table

kvm_pgtable_hyp_unmap() relies on the ->page_count() function callback
being provided by the memory-management operations for the page-table.

Wire up this callback for the hypervisor stage-1 page-table.

Signed-off-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/20211215161232.1480836-5-qperret@google.com