target/mips: Introduce decodetree helpers for MSA LSA/DLSA opcodes

Add the LSA opcode to the MSA32 decodetree config, add DLSA
to a new config for the MSA64 ASE, and call decode_msa64()
in the main decode_opc() loop.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20201215225757.764263-23-f4bug@amsat.org>