Documentation: DT: Add bindings for FSL NS16550A UART

This patch addss the device-tree documentation for Freescale's
NS16550 UART (also called DUART).

There is a specific errata fix required in FSL NS16550 UART
which ensures that an random interrupt storm is not observed when
a break is provided as an input to the UART.
Signed-off-by: Bhupesh Sharma <bhupesh.sharma@freescale.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Olof Johansson <olof@lixom.net>

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

@@ -16,6 +16,7 @@ Required properties:
 	- "altr,16550-FIFO64"
 	- "altr,16550-FIFO128"
 	- "fsl,16550-FIFO64"
+	- "fsl,ns16550"
 	- "serial" if the port type is unknown.
 - reg : offset and length of the register set for the device.
 - interrupts : should contain uart interrupt.
@@ -40,6 +41,17 @@ Optional properties:
   driver is allowed to detect support for the capability even without this
   property.
 
+Note:
+* fsl,ns16550:
+  ------------
+  Freescale DUART is very similar to the PC16552D (and to a
+  pair of NS16550A), albeit with some nonstandard behavior such as
+  erratum A-004737 (relating to incorrect BRK handling).
+
+  Represents a single port that is compatible with the DUART found
+  on many Freescale chips (examples include mpc8349, mpc8548,
+  mpc8641d, p4080 and ls2085a).
+
 Example:
 
 	uart@80230000 {