powerpc64: cpu_initclocks: do tc_init(9) before cpu_startclock()

powerpc64: cpu_initclocks: do tc_init(9) before cpu_startclock()

In the future, the clock interrupt will need a working timecounter to
accurately reschedule itself.

Move tc_init(9) up before cpu_startclock().

kettenis@ notes several other platforms need this same change.  Maybe
we can do the rest all at once.

Tested by kettenis@.

Link: https://marc.info/?l=openbsd-tech&m=165343754512382&w=2
ok kettenis@