watchdog: octeon: use fixed length string for register names

Use fixed length string for register names. This saves 416 bytes
in text size.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>