dt-bindings: watchdog: Add Nuvoton NPCM description

These bindings describe the watchdog IP as used by the Nuvoton NPCM750
(Poleg) BMC SoC.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>