MPC83xx SPISEL_BOOT gpio controller

Provide access to MPC83xx SPISEL_BOOT signal as a gpio to allow it to be
easily bound as a SPI controller chip select.

The SPISEL_BOOT signal is always an output.

Required properties:

- compatible: must be "fsl,mpc83xx-spisel-boot" or "fsl,mpc8309-spisel-boot".
- reg: must point to the SPI_CS register in the SoC register map.
- ngpios: number of gpios provided by driver, normally 1.

Example:

	spisel_boot: spisel_boot@14c {
		compatible = "fsl,mpc8309-spisel-boot";
		reg = <0x14c 0x04>;
		#gpio-cells = <2>;
		device_type = "gpio";
		ngpios = <1>;
	};