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

arch/loongarch/include/asm/loongarch.h

@@ -423,9 +423,9 @@ static __always_inline void iocsr_write64(u64 val, u32 reg)
 #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 */