LoongArch: Replace hard-coded values in comments with VALEN

According to LoongArch documentation [1], CSR.PGDL and CSR.PGDH are
concerned with the VA's MSB which is VALEN-1 instead of always being 47.
Fix comments to avoid misleading others.

[1] https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html#page-global-directory-base-address-for-lower-half-address-space



Reviewed-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Enze Li <lienze@kylinos.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>