target/mips: Introduce decodetree structure for NEC Vr54xx extension

The decoder is called but doesn't decode anything. This will
ease reviewing the next commit.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210801235926.3178085-3-f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
5 files changed