ACPICA: Cleanup PM_TIMER_FREQUENCY definition.

This is a cosmetic patch only. Comparison of the resulting binary showed
only line number differences.

This patch does not affect the generation of the Linux binary.
This patch decreases 13 lines of 20121114 divergence.diff.

There is updates in ACPICA for PM_TIMER_FREQUENCY macro, this patch cleans
up the usage of this macro in Linux. This patch can also reduce the source
code diff between Linux and ACPICA.

Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>