sched/clock, x86: Move some cyc2ns() code around

There are no __cycles_2_ns() users outside of arch/x86/kernel/tsc.c,
so move it there.

There are no cycles_2_ns() users.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-01lslnavfgo3kmbo4532zlcj@git.kernel.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>