dt: document the of_serial bindings

The of_serial bindings can be used to register a number of serial
devices.  Document this binding with all of the others.

v3: remove device-type and clarify used-by-rtas
Signed-off-by: NJamie Iles <jamie@jamieiles.com>
Acked-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>

Documentation/devicetree/bindings/tty/serial/of-serial.txt

+* UART (Universal Asynchronous Receiver/Transmitter)
+
+Required properties:
+- compatible : one of:
+	- "ns8250"
+	- "ns16450"
+	- "ns16550a"
+	- "ns16550"
+	- "ns16750"
+	- "ns16850"
+	- "nvidia,tegra250-uart"
+	- "ibm,qpace-nwp-serial"
+	- "serial" if the port type is unknown.
+- reg : offset and length of the register set for the device.
+- interrupts : should contain uart interrupt.
+- clock-frequency : the input clock frequency for the UART.
+
+Optional properties:
+- current-speed : the current active speed of the UART.
+- reg-offset : offset to apply to the mapbase from the start of the registers.
+- reg-shift : quantity to shift the register offsets by.
+- used-by-rtas : set to indicate that the port is in use by the OpenFirmware
+  RTAS and should not be registered.
+
+Example:
+
+	uart@80230000 {
+		compatible = "ns8250";
+		reg = <0x80230000 0x100>;
+		clock-frequency = <3686400>;
+		interrupts = <10>;
+		reg-shift = <2>;
+	};