MFC r261342

Add hwpmc(4) support for the PowerPC 970 class processors, direct events.
This also fixes asserts on removal of the module for the mpc74xx.

The PowerPC 970 processors have two different types of events: direct events
and indirect events.  Thus far only direct events are supported.  I included
some documentation in the driver on how indirect events work, but support is
for the future.