KVM: arm64: Add PKVM_HYP_REQ_SPLIT

With the upcoming support for stage-2 huge mappings for protected VMs,
we need a way to split blocks. Since the host has its own "copy" of the
guest stage-2 in the pkvm_mappings rb-tree, the split must be done
simultaneously for both that tree and the guest stage-2. Therefore the
hypervisor can't do it on its own and must rely on the host for this
operation.

Create a pKVM hypervisor request to ask the host to split a specified
region of the guest. On this request, the host can synchronise the split
of both guest stage-2 (HVC __pkvm_host_split_guest) and the
pkvm_mappings tree. It ensures a concurrent VM teardown can't observe a
PMD_SIZE pkvm_mapping while the guest stage-2 is PAGE_SIZE.

Signed-off-by: Vincent Donnefort <vdonnefort@google.com>