drivers: serial: add Renesas R-Car driver

This patch add support for polling based UART
on the Renesas R-Car SCIF (Serial Communication Interface
with FIFO)

This hardware block can be found on various Renesas R-Car
SoC series.

It allows to get console on R-Car Gen3 H3ULCB board.

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