armv7m: update to use correct register core_cache

The was missed when the armv7m was moved over to using the std arm
core_cache, probably because it is disabled by default.

Change-Id: I2f5a18ef6dd783b36e8c29f4c52379104bda4583
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1138
Tested-by: jenkins

src/target/armv7m.c

@@ -289,7 +289,7 @@ int armv7m_get_gdb_reg_list(struct target *target, struct reg **reg_list[], int
 	 * if it does not support this arch */
 	*((char *)armv7m->arm.pc->value) |= 1;
 #else
-	(*reg_list)[25] = &armv7m->core_cache->reg_list[ARMV7M_xPSR];
+	(*reg_list)[25] = &armv7m->arm.core_cache->reg_list[ARMV7M_xPSR];
 #endif
 
 	return ERROR_OK;