Maxim thermocouple support

* https://datasheets.maximintegrated.com/en/ds/MAX6675.pdf
* https://datasheets.maximintegrated.com/en/ds/MAX31855.pdf

Required properties:

	- compatible: must be "maxim,max31855" or "maxim,max6675"
	- reg: SPI chip select number for the device
	- spi-max-frequency: must be 4300000
	- spi-cpha: must be defined for max6675 to enable SPI mode 1

	Refer to spi/spi-bus.txt for generic SPI slave bindings.

Example:

	max31855@0 {
		compatible = "maxim,max31855";
		reg = <0>;
		spi-max-frequency = <4300000>;
	};