* Generic 8-bits shift register GPIO driver

Required properties:
- compatible: Should contain one of the following:
    "fairchild,74hc595"
    "nxp,74lvc594"
- reg : chip select number
- gpio-controller : Marks the device node as a gpio controller.
- #gpio-cells : Should be two.  The first cell is the pin number and
  the second cell is used to specify the gpio polarity:
      0 = active high
      1 = active low
- registers-number: Number of daisy-chained shift registers

Example:

gpio5: gpio5@0 {
	compatible = "fairchild,74hc595";
	reg = <0>;
	gpio-controller;
	#gpio-cells = <2>;
	registers-number = <4>;
	spi-max-frequency = <100000>;
};