hw/mips: Merge 'hw/mips/cpudevs.h' with 'target/mips/cpu.h'

"hw/mips/cpudevs.h" contains declarations which are specific
to the MIPS architecture; it doesn't make sense for these to
be called from a non-MIPS architecture. Move the declarations
to "target/mips/cpu.h".

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20231009171443.12145-2-philmd@linaro.org>