Microchip MCP2308/MCP23S08/MCP23017/MCP23S17 driver for
8-/16-bit I/O expander with serial interface (I2C/SPI)

Required properties:
- compatible : Should be
    - "mcp,mcp23s08" for  8 GPIO SPI version
    - "mcp,mcp23s17" for 16 GPIO SPI version
    - "mcp,mcp23008" for  8 GPIO I2C version or
    - "mcp,mcp23017" for 16 GPIO I2C version of the chip
- #gpio-cells : Should be two.
  - first cell is the pin number
  - second cell is used to specify flags. Flags are currently unused.
- gpio-controller : Marks the device node as a GPIO controller.
- reg : For an address on its bus. I2C uses this a the I2C address of the chip.
        SPI uses this to specify the chipselect line which the chip is
        connected to. The driver and the SPI variant of the chip support
        multiple chips on the same chipselect. Have a look at
        mcp,spi-present-mask below.

Required device specific properties (only for SPI chips):
- mcp,spi-present-mask : This is a present flag, that makes only sense for SPI
        chips - as the name suggests. Multiple SPI chips can share the same
        SPI chipselect. Set a bit in bit0-7 in this mask to 1 if there is a
        chip connected with the corresponding spi address set. For example if
        you have a chip with address 3 connected, you have to set bit3 to 1,
        which is 0x08. mcp23s08 chip variant only supports bits 0-3. It is not
        possible to mix mcp23s08 and mcp23s17 on the same chipselect. Set at
        least one bit to 1 for SPI chips.
- spi-max-frequency = The maximum frequency this chip is able to handle

Example I2C:
gpiom1: gpio@20 {
        compatible = "mcp,mcp23017";
        gpio-controller;
        #gpio-cells = <2>;
        reg = <0x20>;
};

Example SPI:
gpiom1: gpio@0 {
        compatible = "mcp,mcp23s17";
        gpio-controller;
        #gpio-cells = <2>;
        spi-present-mask = <0x01>;
        reg = <0>;
        spi-max-frequency = <1000000>;
};