hyperv: Register the MSR-based timecounter during SI_SUB_HYPERVISOR

This reverts commit 9ef7df022a46 ("hyperv: Register hyperv_timecounter
later during boot") and adds a comment explaining why the timecounter
needs to be registered as early as it is.

PR: 259878
Fixes: 9ef7df022a46 ("hyperv: Register hyperv_timecounter later during boot")
Reviewed by: kib
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33014