* Serial-in/parallel-out 8-bits 74hc595 shift register GPIO driver

Required properties:
- compatible : Should be "avm,74hc595"
- 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
- shcp-gpios: Shift register clock pin
- ds-gpios: Shift register serial data in pin
- stcp-gpios: Shift register storage register clock pin
- num-registers: Number of daisy-chained shift registers (each register
  provides 8 GPOs)

Example:

avm_gpio_74hc595: avm-gpio-74hc595 {
    compatible = "avm,74hc595";
    gpio-controller;
    #gpio-cells = <2>;
    shcp-gpios = <&tlmm 46 GPIO_ACTIVE_LOW>;
    ds-gpios = <&tlmm 18 GPIO_ACTIVE_LOW>;
    stcp-gpios = <&tlmm 40 GPIO_ACTIVE_LOW>;
    num-registers = <1>;
};

Usage of GPIOs from the shift register:

gpios = <&avm_gpio_74hc595 0 GPIO_ACTIVE_HIGH>;