target: constify structures

Change-Id: I875cfab8dec4ade72ed9c9cd7d52baaca182a1ef
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/2295
Tested-by: jenkins

src/target/dsp563xx_once.h

@@ -63,9 +63,9 @@
 #define DSP563XX_ONCE_NOREG	0x01F	/* no register selected */
 
 struct once_reg {
-	uint8_t num;
-	uint8_t addr;
-	uint8_t len;
+	const uint8_t num;
+	const uint8_t addr;
+	const uint8_t len;
 	const char *name;
 	uint32_t reg;
 };