ARM: renesas: Enable serial RX buffer on Renesas R-Car

ARM: renesas: Enable serial RX buffer on Renesas R-Car

Enable CONFIG_SERIAL_RX_BUFFER on all Renesas R-Car devices which
use the SCIF serial port. This allows receiving large strings at
bulk even if the RX FIFO is small and would otherwise overflow.

The usual trigger for the problem addressed here is a paste of a
very long command into U-Boot command line, somewhere between 400
and 500 characters long. The trailing end of the string is usually
not received and the command line input stops responding due to RX
overflow errors. The CONFIG_SERIAL_RX_BUFFER allows efficient read
of data from the RX FIFO, which prevents the overflow, and allows
safe reception of long pasted strings.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>