[v2] P9 ldmx support

[Jack Miller]

Second spin of these patches:

https://lists.ozlabs.org/pipermail/linuxppc-dev/2016-April/141609.html

Differences from v1:

- As part of the FSCR context switch patch, remove extra FSCR manipulation in
  the DSCR case. If anything set FSCR.DSCR it should automatically be set
  correctly now. If I understand correctly, dscr_inherit is still required
  however and covers the < P8 case.

- Minor assembly readability clean up suggested by Segher (thanks!)

Any further comments or concerns welcome.

- Jack