parisc: Mark sched_clock unstable only if clocks are not syncronized

We check at runtime if the cr16 clocks are stable across CPUs. Only mark
the sched_clock unstable by calling clear_sched_clock_stable() if we
know that we run on a system which isn't syncronized across CPUs.

Signed-off-by: Helge Deller <deller@gmx.de>