Merge remote-tracking branch 'remotes/xtensa/tags/20150706-xtensa' into staging

Xtensa fixes:

- add 64-bit floating point registers;
- fix gdb register map construction.

# gpg: Signature made Mon Jul  6 11:27:45 2015 BST using RSA key ID F83FA044
# gpg: Good signature from "Max Filippov <max.filippov@cogentembedded.com>"
# gpg:                 aka "Max Filippov <jcmvbkbc@gmail.com>"

* remotes/xtensa/tags/20150706-xtensa:
  target-xtensa: fix gdb register map construction
  target-xtensa: add 64-bit floating point registers

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>