watchdog: imgpdc: Fix max timeout

Maximum timeout is currently set in clock cycles, but the watchdog
core expects it to be in seconds. Fix it.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>