throttling, neglecting to do this kept the sysctls from appearing.
Attach an acpi_throttle device to each CPU that supports it.
Don't add a device if the P_BLK is invalid or if _PTC is not present.
This removes extraneous probe/attach failure messages on some machines.
Make the cpu throttle state local to the softc to account for partial
successes when changing the clock rate on MP machines.