dt-bindings: serial: Document Freescale LINFlexD UART

Add documentation for the serial communication interface module (LINFlexD),
found in two instances on S32V234.
Signed-off-by: Stoica Cosmin-Stefan <cosmin.stoica@nxp.com>
Signed-off-by: Larisa Grigore <Larisa.Grigore@nxp.com>
Signed-off-by: Stefan-Gabriel Mirea <stefan-gabriel.mirea@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20190823191115.18490-7-stefan-gabriel.mirea@nxp.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/fsl,s32-linflexuart.txt

+* Freescale LINFlexD UART
+
+The LINFlexD controller implements several LIN protocol versions, as well as
+support for full-duplex UART communication through 8-bit and 9-bit frames.
+
+See chapter 47 ("LINFlexD") in the reference manual[1].
+
+Required properties:
+- compatible :
+  - "fsl,s32v234-linflexuart" for LINFlexD configured in UART mode, which
+    is compatible with the one integrated on S32V234 SoC
+- reg : Address and length of the register set for the device
+- interrupts : Should contain uart interrupt
+
+Example:
+uart0: serial@40053000 {
+	compatible = "fsl,s32v234-linflexuart";
+	reg = <0x0 0x40053000 0x0 0x1000>;
+	interrupts = <0 59 4>;
+};
+
+[1] https://www.nxp.com/webapp/Download?colCode=S32V234RM