x86: unify timex.h variants

Combine the timex.h variants and move the TSC related code into tsc.h.
Move the set_cyc2ns_scale() call into the tsc calibraction code, where
it belongs.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>