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>
 #define  CSR_ASID_ASID_WIDTH           10
 #define  CSR_ASID_ASID                 (_ULCAST_(0x3ff) << CSR_ASID_ASID_SHIFT)
 
-#define LOONGARCH_CSR_PGDL             0x19    /* Page table base address when VA[47] = 0 */
+#define LOONGARCH_CSR_PGDL             0x19    /* Page table base address when VA[VALEN-1] = 0 */
 
-#define LOONGARCH_CSR_PGDH             0x1a    /* Page table base address when VA[47] = 1 */
+#define LOONGARCH_CSR_PGDH             0x1a    /* Page table base address when VA[VALEN-1] = 1 */
 
 #define LOONGARCH_CSR_PGD              0x1b    /* Page table base */