hwmon: (nct7904) Add watchdog function

Implement watchdog functionality for NCT7904.

Signed-off-by: Yuechao Zhao <yuechao.zhao@advantech.com.cn>
Link: https://lore.kernel.org/r/20200331052850.5419-1-yuechao.zhao@advantech.com.cn


[groeck: Squashed fixup patch]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>