iio:temperature:max31856:Add device tree bind info

This patch added device tree binding info for MAX31856 driver.
Signed-off-by: Paresh Chaudhary <paresh.chaudhary@rockwellcollins.com>
Signed-off-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Patrick Havelange <patrick.havelange@essensium.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/temperature/max31856.txt

+Maxim MAX31856 thermocouple support
+
+https://datasheets.maximintegrated.com/en/ds/MAX31856.pdf
+
+Optional property:
+	- thermocouple-type: Type of thermocouple (THERMOCOUPLE_TYPE_K if
+		omitted). Supported types are B, E, J, K, N, R, S, T.
+
+Required properties:
+	- compatible: must be "maxim,max31856"
+	- reg: SPI chip select number for the device
+	- spi-max-frequency: As per datasheet max. supported freq is 5000000
+	- spi-cpha: must be defined for max31856 to enable SPI mode 1
+
+	Refer to spi/spi-bus.txt for generic SPI slave bindings.
+
+ Example:
+	temp-sensor@0 {
+		compatible = "maxim,max31856";
+		reg = <0>;
+		spi-max-frequency = <5000000>;
+		spi-cpha;
+		thermocouple-type = <THERMOCOUPLE_TYPE_K>;
+	};