target/mips: Check nanoMIPS DSP MULT[U] accumulator with Release 6

target/mips: Check nanoMIPS DSP MULT[U] accumulator with Release 6

Per the "MIPS Architecture Extension: nanoMIPS32 DSP TRM" rev 0.04,
MULT and MULTU opcodes:

  The value of ac selects an accumulator numbered from 0 to 3.
  When ac=0, this refers to the original HI/LO register pair of the
  MIPS32 architecture.

  In Release 6 of the MIPS Architecture, accumulators are eliminated
  from MIPS32.

Ensure pre-Release 6 is restricted to HI/LO registers pair.

Fixes: 8b3698b2947 ("target/mips: Add emulation of DSP ASE for nanoMIPS - part 4")
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>