serial: sh-sci: Add (H)SCIF RTS/CTS pin data register bit definitions

Add the missing register bit definitions to set the RTS pin and read the
CTS pin on (H)SCIF.

Extracted from patches by Magnus Damm <damm+renesas@opensource.se>.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>