Microsemi Corporation (MSCC) Serial GPIO driver

The MSCC serial GPIO extends the number or GPIO's on the system by
means of 4 dedicated pins: one input, one output, one clock and one
strobe pin. By attaching a number of (external) shift registers, the
effective GPIO count can be extended by up to 128 GPIO's per
controller.

Required properties:
- compatible : "mscc,luton-sgpio" or "mscc,ocelot-sgpio"
- clock: Reference clock used to generate clock divider setting. See
  mscc,sgpio-frequency property.
- reg : Physical base address and length of the controller's registers.
- #gpio-cells : Should be two. The first cell is the pin number and the
  second cell is used to specify optional parameters:
  - bit 0 specifies polarity (0 for normal, 1 for inverted)
- gpio-controller : Marks the device node as a GPIO controller.
- gpio-ranges: Standard gpio range(s): phandle, gpio base, pinctrl base
  and count.

Optional properties:
- ngpios: See gpio.txt
- mscc,sgpio-frequency: The frequency at which the serial bitstream is
  generated and sampled. Default: 12500000 (Hz).
- mscc,sgpio-ports: A bitmask (32 bits) of which ports are enabled in
  the serialized gpio stream. One 'port' will transport from 1 to 4
  gpio bits. Default: 0xFFFFFFFF.

Typically the pinctrl-0 and pinctrl-names properties will also be
present to enable the use of the SIO CLK, LD, DI and DO for some
regular GPIO pins.

Example:

sgpio: gpio@10700f8 {
       compatible = "mscc,ocelot-sgpio";
       pinctrl-0 = <&sgpio_pins>;
       pinctrl-names = "default";
       reg = <0x10700f8 0x100>;
       gpio-controller;
       #gpio-cells = <2>;
       gpio-ranges = <&sgpio 0 0 64>;
       mscc,sgpio-frequency = <12500>;
       mscc,sgpio-ports = <0x000FFFFF>;
};