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>;
	};