x86: Rename the _D dirty flag

This value happens to be used by ctype.h so chose a different name.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/x86/cpu/i386/cpu.c b/arch/x86/cpu/i386/cpu.c
index 845e00c..b108f72 100644
--- a/arch/x86/cpu/i386/cpu.c
+++ b/arch/x86/cpu/i386/cpu.c
@@ -596,7 +596,7 @@
 #define _RW   BIT(1)	/* write allowed */
 #define _US   BIT(2)	/* user-access allowed */
 #define _A    BIT(5)	/* has been accessed */
-#define _D    BIT(6)	/* has been written to */
+#define _DT   BIT(6)	/* has been written to */
 #define _PS   BIT(7)	/* indicates 2MB page size here */
 
 /**
@@ -620,7 +620,7 @@
 
 	/* Level 2 has 2048 64-bit entries, each repesenting 2MiB */
 	for (i = 0; i < 2048; i++)
-		pgtable[2048 + i * 2] = _PRES + _RW + _US + _PS + _A +  _D +
+		pgtable[2048 + i * 2] = _PRES + _RW + _US + _PS + _A +  _DT +
 					 (i << 21UL);
 }