first version of my decoder function skeleton

first version of my decoder function skeleton
- this will decode the instructions but not doing anything to create the C++ object yet
(the 1st of many steps!)

arch/mips/isa_desc/bitfields.h:
    initial bitfield constants ... copied some from original alpha bitfields
arch/mips/isa_desc/decoder.h:
    decoder function skeleton pt.1
    - this will decode the instructions but not doing anything to create the C++ object yet
    (the 1st of many steps!)

--HG--
extra : convert_revision : 2b9a0f8160c78b17f9d3d5eaf5af5a4d2f074761