More from Bruno Ducrot.

Warning: it will only half work on the ACER Aspire, though: there is
no pstate in the DSDT corresponding to the max frequency...
I'm looking how to handle that correctly, probably at the init stage,
if the max frequency is not given, then add it to powernow_table.

* Integrate acpi perflib from Dominik.
* Use acpi if the PST tables are known to be broken (ASUS Aspire match one PST,
  but give brain damaged values), or use ACPI if no PST found.