drivers/at91rm9200: fix help and usage to command "at91rm9200_device"

Add usage and fix the help text of the command

Change-Id: I0e157d76f5d87bcda80f82a110912994908c8e03
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5023Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins

src/jtag/drivers/at91rm9200.c

@@ -183,7 +183,8 @@ static const struct command_registration at91rm9200_command_handlers[] = {
 		.name = "at91rm9200_device",
 		.handler = &at91rm9200_handle_device_command,
 		.mode = COMMAND_CONFIG,
-		.help = "query armjtagew info",
+		.help = "Set at91rm9200 device [default \"rea_ecr\"]",
+		.usage = "<device>",
 	},
 	COMMAND_REGISTRATION_DONE
 };