Add TableGen info for Power8.
This is based on the Power7 version, with units added and renamed to match P8.

Differential Revision: http://reviews.llvm.org/D6358




git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223257 91177308-0d34-0410-b5e6-96231b3b80d8