[mips] IIImult should have an InstrItinData in the generic scheduler. Used the same one as for IIImul.

Affects:
  DMULT, DMULTu, MADD, MADD_MM, MADDU, MADDU_MM, MSUB, MSUB_MM, MSUBU,
  MSUBU_MM, MULT, MULTu

Does not affect MULT_MM, MULTu_MM since they are currently miscategorised
as IIImul.
Giving the asm attribute some keyword spellings based off of GCC's documentation. No functional changes.