Serial Slave Device DT binding

This documents the binding structure and common properties for serial
attached devices. Common examples include Bluetooth, WiFi, NFC and GPS
devices.

Serial attached devices shall be a child node of the host UART device the
slave device is attached to. It is expected that the attached device is
the only child node of the UART device. The slave device node name shall
reflect the generic type of device for the node.

Required Properties:

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

Optional Properties:

- max-speed	: The maximum baud rate the device operates at. This should
		  only be present if the maximum is less than the slave device
		  can support. For example, a particular board has some signal
		  quality issue or the host processor can't support higher
		  baud rates.
- current-speed	: The current baud rate the device operates at. This should
		  only be present in case a driver has no chance to know
		  the baud rate of the slave device.
		  Examples:
		    * device supports auto-baud
		    * the rate is setup by a bootloader and there is no
		      way to reset the device
		    * device baud rate is configured by its firmware but
		      there is no way to request the actual settings

Example:

serial@1234 {
	compatible = "ns16550a";
	interrupts = <1>;

	bluetooth {
		compatible = "brcm,bcm43341-bt";
		interrupt-parent = <&gpio>;
		interrupts = <10>;
	};
};