watchdog: npcm: Enable modular builds

Most of the NPCM7XX drivers can be built as modules.
The NPCM7XX watchdog driver code supports building it as
module, but its configuration option is set to bool.
Make itr tristate to actually support modular builds.
This improves consistency with other drivers for the
same platform and enables including the driver in multi-
platform configurations.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>