hwmon: Define binding for the nsa320-hwmon driver

Define a binding for the hardware monitoring chip present in the Zyxel
NSA-320 and some of the other Zyxel NAS devices.

Signed-off-by: Adam Baker <linux@baker-net.org.uk>
Acked-by: Rob Herring <robh@kernel.org>
[groeck: Fixed whitespace error]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>