Number of ALU in la464.md

[Lulu Cheng]

You are right, I just added it before, but I didn't see the performance 
improvement,

so I didn't change this place.

在 2022/9/25 下午7:28, Xi Ruoyao 写道:
> Hi,
>
> According to the schematic
> (https://loongson.github.io/LoongArch-Documentation/Loongson-3A5000-usermanual-EN.html#la464-processor-core,
> Figure 5) we have 4 ALUs in LA464, but in la464.md we only define
> la464_alu1 and la464_alu2.
>
> Should we add la464_alu{3,4} as well, or am I missing something here?