target/mips: Remove vendor specific CPU definitions

Vendor specific CPU definitions are not very useful. Use the
ISA definitions instead, which are more helpful when looking
at the various CPU definitions.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210112210152.2072996-4-f4bug@amsat.org>