ANDROID: KVM: arm64: Add support for custom hypercall registration

When pKVM is in use, allow pKVM modules to register custom hypercall
handlers:

   * pkvm_register_el2_call(): Give a handler to the hypervisor and gets in
     return the newly registered hypercall number.

   * pkvm_el2_mod_call(): Call the previously registered hypercall handler.

There is a limit of 128 hypercalls that can be registered.

Bug: 278749606
Bug: 244543039
Bug: 244373730
Change-Id: I3d6c89675efe5f65f6b53c3b45ae155d1a00164c
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Quentin Perret <qperret@google.com>