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.debianSigned-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/devicetree/bindings/hwmon/as370.txt

+Bindings for Synaptics AS370 PVT sensors
+
+Required properties:
+- compatible : "syna,as370-hwmon"
+- reg        : address and length of the register set.
+
+Example:
+	hwmon@ea0810 {
+		compatible = "syna,as370-hwmon";
+		reg = <0xea0810 0xc>;
+	};