arm: Use different enum for core_type and core_mode

The fields core_type and core_mode use the same enum arm_mode
but encode different information, making the code less immediate
to read.

Use a different enum arm_core_type for the field core_type.
The code behavior is not changed.

Change-Id: I60f2095ea6801dfe22f6da81ec295ca71ef90466
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5258
Tested-by: jenkins

src/target/cortex_a.c

@@ -2787,7 +2787,7 @@ static int cortex_a_examine_first(struct target *target)
 		}
 	}
 
-	armv7a->arm.core_type = ARM_MODE_MON;
+	armv7a->arm.core_type = ARM_CORE_TYPE_SEC_EXT;
 
 	/* Avoid recreating the registers cache */
 	if (!target_was_examined(target)) {