watchdog: imgpdc: Allow timeout to be set in device-tree

Since the heartbeat is statically initialized to its default value,
watchdog_init_timeout() will never look in the device-tree for a
timeout-sec value.  Instead of statically initializing heartbeat,
fall back to the default timeout value if watchdog_init_timeout()
fails.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Reviewed-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Tested-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Cc: James Hogan <james.hogan@imgtec.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>