* UART (Universal Asynchronous Receiver/Transmitter)

Required properties:
- compatible : one of:
	- "ns8250"
	- "ns16450"
	- "ns16550a"
	- "ns16550"
	- "ns16750"
	- "ns16850"
	- "nvidia,tegra20-uart"
	- "nxp,lpc3220-uart"
	- "ibm,qpace-nwp-serial"
	- "altr,16550-FIFO32"
	- "altr,16550-FIFO64"
	- "altr,16550-FIFO128"
	- "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
	 or
  clocks phandle to refer to the clk used as per Documentation/devicetree
  /bindings/clock/clock-bindings.txt

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.
- reg-io-width : the size (in bytes) of the IO accesses that should be
  performed on the device.  There are some systems that require 32-bit
  accesses to the UART (e.g. TI davinci).
- used-by-rtas : set to indicate that the port is in use by the OpenFirmware
  RTAS and should not be registered.
- no-loopback-test: set to indicate that the port does not implements loopback
  test mode

Example:

	uart@80230000 {
		compatible = "ns8250";
		reg = <0x80230000 0x100>;
		clock-frequency = <3686400>;
		interrupts = <10>;
		reg-shift = <2>;
	};