ANDROID: KVM: arm64: Allow hyp_pool to be initialized without pages

Currently, hyp_pool_init requires pfn to initialize the free lists,
with the nr_pages, this doesn't fit in the IOMMU model, where we
want to init the hyp pool without pages yet, and the pool will be toped
in runtime when pages are reclaimed then re-used for next allocations.
A new bool arg(empty_alloc) is added to allow this behavior.

Bug: 277989609
Bug: 278749606
Change-Id: If5710a740f5078e6511631a0cd19c2ed39605f18
Signed-off-by: Mostafa Saleh <smostafa@google.com>