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/cfi.c

@@ -3112,7 +3112,7 @@ static void cfi_fixup_0002_write_buffer(struct flash_bank *bank, const void *par
 	cfi_info->buf_write_timeout_typ = 0;
 }
 
-struct flash_driver cfi_flash = {
+const struct flash_driver cfi_flash = {
 	.name = "cfi",
 	.flash_bank_command = cfi_flash_bank_command,
 	.erase = cfi_erase,