ANDROID: KVM: arm64: Move enum pkvm_page_state to memory.h

In order to prepare the way for storing page-tracking information in the
vmemmap, move the enum pkvm_page_state definition to nvhe/memory.h.

No functional changes intended.

Bug: 357781595
Bug: 322736772
Bug: 278749606
Bug: 326193089
Change-Id: I6f8fa9004ccbc9b65f36d60889cdaea651b228ec
Signed-off-by: Quentin Perret <qperret@google.com>