KVM: arm64: nv: Turn upper_attr for S2 walk into the full descriptor

The upper_attr attribute has been badly named, as it most of the
time carries the full "last walked descriptor".

Rename it to "desc" and make ti contain the full 64bit descriptor.
This will be used by the S1 PTW.

Signed-off-by: Marc Zyngier <maz@kernel.org>