hwmon: (as370-hwmon) Add DT bindings for Synaptics AS370 PVT

Add device tree bindings for Synaptics AS370 PVT sensors.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20190827113337.384457f6@xhacker.debian
Signed-off-by: Guenter Roeck <linux@roeck-us.net>