Merge remote-tracking branch 'remotes/xtensa/tags/20200106-xtensa' into staging

Merge remote-tracking branch 'remotes/xtensa/tags/20200106-xtensa' into staging

target/xtensa improvements for v5.0:

- fix ps.ring use in MPU configs;
- use MPU background map from the configuration overlay.

# gpg: Signature made Mon 06 Jan 2020 20:00:33 GMT
# gpg:                using RSA key 2B67854B98E5327DCDEB17D851F9CC91F83FA044
# gpg:                issuer "jcmvbkbc@gmail.com"
# gpg: Good signature from "Max Filippov <filippov@cadence.com>" [unknown]
# gpg:                 aka "Max Filippov <max.filippov@cogentembedded.com>" [full]
# gpg:                 aka "Max Filippov <jcmvbkbc@gmail.com>" [full]
# Primary key fingerprint: 2B67 854B 98E5 327D CDEB  17D8 51F9 CC91 F83F A044

* remotes/xtensa/tags/20200106-xtensa:
  target/xtensa: use MPU background map from core configuration
  target/xtensa: import xtensa/config/core-isa.h
  target/xtensa: fix ps.ring use in MPU configs

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