target/xtensa updates:

- add libisa to the xtensa target;
- change xtensa instruction translator to use it;
- switch existing xtensa cores to use it;
- add support for a number of instructions: salt/saltu, const16,
  GPIO32 group, debug mode and MMU-related;
- add disassembler for Xtensa.

# gpg: Signature made Tue 09 Jan 2018 18:11:02 GMT
# gpg:                using RSA key 0x51F9CC91F83FA044
# gpg: Good signature from "Max Filippov <filippov@cadence.com>"
# gpg:                 aka "Max Filippov <max.filippov@cogentembedded.com>"
# gpg:                 aka "Max Filippov <jcmvbkbc@gmail.com>"
# Primary key fingerprint: 2B67 854B 98E5 327D CDEB  17D8 51F9 CC91 F83F A044

* remotes/xtensa/tags/20180109-xtensa:
  target/xtensa: implement disassembler
  target/xtensa: implement const16
  target/xtensa: implement GPIO32
  target/xtensa: implement salt/saltu
  target/xtensa: add internal/noop SRs and opcodes
  target/xtensa: drop DisasContext::litbase
  target/xtensa: tests: fix memctl SR test
  target/xtensa: use libisa for instruction decoding
  target/xtensa: switch fsf to libisa
  target/xtensa: switch dc233c to libisa
  target/xtensa: switch dc232b to libisa
  target/xtensa: update import_core.sh script for libisa
  target/xtensa: extract FPU2000 opcode translators
  target/xtensa: extract core opcode translators
  target/xtensa: import libisa source
  target/xtensa: pass actual frame size to the entry helper

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