aarch64: stub new mmu functions in bsd

commit 82f881a2 "zfs: mmu: take vma_list_mutex"
introduces code that needs to be stubbed for now for AArch64.

Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>