target/mips: Disable DSP ASE for Octeon68XX

I don't have access to Octeon68XX hardware but according
to my investigation Octeon never had DSP ASE support.

As per "Cavium Networks OCTEON Plus CN50XX Hardware Reference
Manual" CP0C3_DSPP is reserved bit and read as 0. Also I do have
access to a Ubiquiti Edgerouter 4 which has Octeon CN7130 processor
and I can confirm CP0C3_DSPP is read as 0 on that processor.

Further more, in linux kernel:
arch/mips/include/asm/mach-cavium-octeon/cpu-feature-overrides.h
cpu_has_dsp is overridden as 0.

So I believe we shouldn't emulate DSP in QEMU as well.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Acked-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Pavel Dovgalyuk <Pavel.Dovgalyuk@ispras.ru>
Message-Id: <20221031132531.18122-4-jiaxun.yang@flygoat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>