Merge remote-tracking branch 'remotes/rth/tags/pull-dt-20190312' into staging

Break out documentation to docs/devel/.
Add support for pattern groups.
Other misc cleanups for multiple decode functions.

# gpg: Signature made Tue 12 Mar 2019 16:59:37 GMT
# gpg:                using RSA key 64DF38E8AF7E215F
# gpg: Good signature from "Richard Henderson <richard.henderson@linaro.org>" [full]
# Primary key fingerprint: 7A48 1E78 868B 4DB6 A85A  05C0 64DF 38E8 AF7E 215F

* remotes/rth/tags/pull-dt-20190312:
  decodetree: Properly diagnose fields overflowing an insn
  decodetree: Prefix extract function names with decode_function
  decodetree: Allow +- to begin a number initializing a field
  decodetree: Produce clean output for an empty input file
  decodetree: Add --static-decode option
  test/decode: Add tests for PatternGroups
  decodetree: Allow grouping of overlapping patterns
  decodetree: Do not unconditionaly return from Pattern.output_code
  decodetree: Ensure build_tree does not include values outside insnmask
  decodetree: Document the usefulness of argument sets
  decodetree: Move documentation to docs/devel/decodetree.rst
  MAINTAINERS: Add scripts/decodetree.py to the TCG section

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