ANDROID: KVM: arm64: Count KVM s2 mmu usage in nVHE protected mode

When using the nVHE protected mode, the stage-2 page tables are handled by
the hypervisor, but are backed by memory donated by the host. That memory
is accounted during the donation (add to the vCPUs hyp_memcache) under
secondary pagetable stats.

Bug: 278749606
Bug: 222044477
Change-Id: I2a45ce65c5ce9cf96aabd1b66d6f83ffe4808a0c
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>