target/mips: Restrict mips_cpu_dump_state() to cpu.c

As mips_cpu_dump_state() is only used once to initialize the
CPUClass::dump_state handler, we can move it to cpu.c to keep
it symbol local.
Beside, this handler is used by all accelerators, while the
translate.c file targets TCG.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210428170410.479308-7-f4bug@amsat.org>