flash/nor/at91samd: set usage for command "set-security"

The only allowed parameter is the string "enable"

Change-Id: Iefdad38f1b4177e4194e58caf2be654357299ded
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5021
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>

src/flash/nor/at91samd.c

@@ -1241,6 +1241,7 @@ static const struct command_registration at91samd_exec_command_handlers[] = {
 			"This makes it impossible to read the Flash contents. "
 			"The only way to undo this is to issue the chip-erase "
 			"command.",
+		.usage = "'enable'",
 	},
 	{
 		.name = "eeprom",