Number of ALU in la464.md

[Xi Ruoyao]

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?
-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University