watchdog: octeon: convert to WATCHDOG_CORE API

Convert OCTEON watchdog to WATCHDOG_CORE API. This enables support
for multiple watchdogs on OCTEON boards.

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>