From: Dominik Brodowski <linux@dominikbrodowski.de>,
      John Stultz <johnstul@us.ibm.com>,
      Dmitry Torokhov

Add the ACPI Powermanagement Timer as x86 kernel timing source.  Unlike the
Time Stamp Counter, it is a reliable timing source which does not get
affected by aggressive powermanagement features like CPU frequency scaling.

Some ideas and some code are based on Arjan van de Ven's implementation for
2.4, and on R.  Byron Moore's drivers/acpi/hardware/hwtimer.c.


We also replace the loop based delay_pmtmr with a TSC based delay_pmtmr,
which resolves a number of issues caused by the loop based delay.  Unsynced
TSCs as well frequency changing TSCs will effect the length of __delay(), but
it seems this method works best.