serial: sh-sci: Add more Serial Port Register documentation

Improve documentation for the (H)SCIF Serial Port Register:
  - Make it clear the RTS and CTS lines are active-low,
  - Document the bits related to the serial port's clock pin.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>