Hyper-V wants to register its MSR-based timecounter during
SI_SUB_HYPERVISOR, before SI_SUB_LOCK, since an emulated 8254 may not be
available. So we cannot use MTX_SYSINIT to initialize the timecounter
This change reverts commit 9ef7df022a46 ("hyperv: Register
hyperv_timecounter later during boot") and adds some comments explaining
why things are the way they are.