GNSS Receiver DT binding

This documents the binding structure and common properties for GNSS receiver
devices.

A GNSS receiver node is a node named "gnss" and typically resides on a serial
bus (e.g. UART, I2C or SPI).

Please refer to the following documents for generic properties:

	Documentation/devicetree/bindings/serial/serial.yaml
	Documentation/devicetree/bindings/spi/spi-bus.txt

Required properties:

- compatible	: A string reflecting the vendor and specific device the node
		  represents

Optional properties:
- lna-supply	: Separate supply for an LNA
- enable-gpios	: GPIO used to enable the device
- timepulse-gpios	: Time pulse GPIO

Example:

serial@1234 {
	compatible = "ns16550a";

	gnss {
		compatible = "u-blox,neo-8";

		vcc-supply = <&gnss_reg>;
		timepulse-gpios = <&gpio0 16 GPIO_ACTIVE_HIGH>;

		current-speed = <4800>;
	};
};