Update MiscReg enum and miscRegFile definition

Update MiscReg enum and miscRegFile definition
update miscReg file access

arch/mips/isa/decoder.isa:
arch/mips/isa_traits.cc:
    update miscRegfile access
arch/mips/isa_traits.hh:
    Update MiscReg enum and miscRegFile definition

--HG--
extra : convert_revision : 9b6b9343d674e1e38e25bb9a4ffe4325142e7424