hyperv: Register hyperv_timecounter later during boot

Previously the MSR-based timecounter was registered during
SI_SUB_HYPERVISOR, i.e., very early during boot, and before SI_SUB_LOCK.
After commit 621fd9dcb2d8 this triggers a panic since the timecounter
list lock is not yet initialized.

The hyperv timecounter does not need to be registered so early, so defer
that to SI_SUB_DRIVERS, at the same time the hyperv TSC timecounter is
registered.

Reported by: whu
Approved by: whu
Fixes: 621fd9dcb2d8 ("timecounter: Lock the timecounter list")
MFC after: 1 week
Sponsored by: The FreeBSD Foundation