watchdog: npcm: Modify npcm watchdog kconfig arch parameter

Modify Nuvoton watchdog Kconfig default supported architecture
name to ARCH_NPCM7XX because ARCH_NPCM750 architecture name
is not supported.

Signed-off-by: Tomer Maimon <tmaimon77@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>