dt-bindings: serial: fsl-linflexuart: convert to json-schema format

Convert the FSL LINFlexD UART binding to json-schema.

Signed-off-by: Chester Lin <clin@suse.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>