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.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>