Updates to decodetree.py for risc-v.

# gpg: Signature made Wed 31 Oct 2018 16:52:07 GMT
# gpg:                using RSA key 64DF38E8AF7E215F
# gpg: Good signature from "Richard Henderson <richard.henderson@linaro.org>"
# Primary key fingerprint: 7A48 1E78 868B 4DB6 A85A  05C0 64DF 38E8 AF7E 215F

* remotes/rth/tags/pull-dt-20181031:
  decodetree: Allow multiple input files
  decodetree: Remove "insn" argument from trans_* expanders
  decodetree: Add !extern flag to argument sets

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