dt-bindings: Add 1027/1029/1031 SPI ADCs as trivial devices

* Maxim 1027/1029/1031 Analog to Digital Converter (ADC)

Required properties:

  - compatible: Should be "maxim,max1027" or "maxim,max1029" or "maxim,max1031"

  - reg: SPI chip select number for the device

Optional properties:

  - interrupts: IRQ line for the ADC

  see: Documentation/devicetree/bindings/interrupt-controller/interrupts.txt

Recommended properties:

- spi-max-frequency: Definition as per

                     Documentation/devicetree/bindings/spi/spi-bus.txt

Example:

adc@0 {

	compatible = "maxim,max1027";

	reg = <0>;

	interrupt-parent = <&gpio5>;

	interrupts = <15 IRQ_TYPE_EDGE_RISING>;

	spi-max-frequency = <1000000>;

};

          - isil,isl68137

            # 5 Bit Programmable, Pulse-Width Modulator

          - maxim,ds1050

            # 10-bit 8 channels 300ks/s SPI ADC with temperature sensor

          - maxim,max1027

            # 10-bit 12 channels 300ks/s SPI ADC with temperature sensor

          - maxim,max1029

            # 10-bit 16 channels 300ks/s SPI ADC with temperature sensor

          - maxim,max1031

            # Low-Power, 4-/12-Channel, 2-Wire Serial, 12-Bit ADCs

          - maxim,max1237

            # PECI-to-I2C translator for PECI-to-SMBus/I2C protocol conversion