Constify struct flash_driver instances

Instances of struct flash_driver are never written to at runtime. For a
small amount of memory saving and also robustness (fewer things for
stray pointer writes to hit), mark them const.

Change-Id: Iadbbbc2fac0976d892699200000c5f02856729f3
Signed-off-by: Christopher Head <chead@zaber.com>
Reviewed-on: http://openocd.zylin.com/4803
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>

src/flash/nor/driver.h

@@ -231,6 +231,6 @@ struct flash_driver {
  * @param name The name of the requested driver.
  * @returns The flash_driver called @c name, or NULL if not found.
  */
-struct flash_driver *flash_driver_find_by_name(const char *name);
+const struct flash_driver *flash_driver_find_by_name(const char *name);
 
 #endif /* OPENOCD_FLASH_NOR_DRIVER_H */