arch/tile: Use separate, better minsec values for clocksource and sched_clock.

We were using the same 5-sec minsec for the clocksource and sched_clock
that we were using for the clock_event_device.  For the clock_event_device
that's exactly right since it has a short maximum countdown time.
But for sched_clock we want to avoid wraparound when converting from
ticks to nsec over a much longer window, so we force a shift of 10.
And for clocksource it seems dodgy to use a 5-sec minsec as well, so we
copy some other platforms and force a shift of 22.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>