dts: bindings: serial: add Renesas R-Car SCIF

Renesas has Serial Communication Interface with
FIFO (SCIF) hardware block which allow asynchronous and
synchronous serial communication. The SCIF has 16-stage FIFO
buffers separately for transmissions and reception.

Signed-off-by: Aymeric Aillet <aymeric.aillet@iot.bzh>