Eckelmann SIOX GPIO bus

Required properties:
- compatible : "eckelmann,siox-gpio"
- din-gpios, dout-gpios, dclk-gpios, dld-gpios: references gpios for the
    corresponding bus signals.

Examples:

        siox {
                compatible = "eckelmann,siox-gpio";
                pinctrl-names = "default";
                pinctrl-0 = <&pinctrl_siox>;

                din-gpios = <&gpio6 11 0>;
                dout-gpios = <&gpio6 8 0>;
                dclk-gpios = <&gpio6 9 0>;
                dld-gpios = <&gpio6 10 0>;
        };
