dt-bindings: Add devicetree bindings to devicetree project

Binding for SIRF GNSS receiver control driver

GPIO pins are toggled to control GNSS receiver power states either to

wake it from sleep or put receiver into sleep mode

Required properties:

- compatible: must be "gnss_sirf"

- #gpio-pins:

      0: GPIO 18

      1: GPIO 87

Example:

	ss5_pwr_ctrl0 {

		compatible = "gnss_sirf";

		pinctrl-0 = <&ss5_pwr_ctrl_rst_on>;

		ssVreset-gpio = <&tlmm 87 1>;

		ssVonoff-gpio = <&tlmm 18 1>;

	};

	ss5_pwr_ctrl_pins: ss5_pwr_ctrl_pins {

		ss5_pwr_ctrl_rst_on: ss5_pwr_ctrl_rst_on {

			mux {

				pins = "gpio87", "gpio18";

				function = "gpio";

			};

			config {

				pins = "gpio87", "gpio18";

				drive-strength = <16>; /* 16 mA */

				bias-pull-up;

				output-high;

			};

		};

		ss5_pwr_ctrl_rst_off: ss5_pwr_ctrl_off {

			mux {

				pins = "gpio87", "gpio18";

				function = "gpio";

			};

			config {

				pins = "gpio87", "gpio18";

				drive-strength = <16>; /* 16 mA */

				bias-pull-up;

				output-high;

			};

		};

	};