arm,kvm: missed rename of MISCREG_HYP in kvm/armv8_cpu.cc

arm,kvm: missed rename of MISCREG_HYP in kvm/armv8_cpu.cc

A recent checkin changed the names of some of the MISCREGs.
One of them was missed and wont compile. This fixes to the new name.

Change-Id: I746f9dea44bc50819a0d30f62dcc3a46380f80c2
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38358
Reviewed-by: Nathanael Premillieu <nathanael.premillieu@huawei.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>