sim: sh: adjust some dsp insn masks

sim: sh: adjust some dsp insn masks

The pmuls encoding is incorrect -- it looks like a copy & paste error
from the padd pmuls variant.  The SuperH software manual covers this.

On the flip side, the manual lists pwsb & pwad as insns that exist,
but no description of what they do, what the insn name means, or the
actual encoding.  Our sim implementation stubs them both out as nops.
Let's mark the fields to avoid unused variable warnings.