serial: sh-sci: Add more Serial Port Control/Data Register documentation

Improve documentation for the SCIFA/SCIFB Serial Port Control and Data
Registers:
  - State clearly that the RTS and CTS lines are active-low,
  - Document the bits related to the serial port's SCK, RXD, and TXD
    pins.
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: NPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sh-sci.h

@@ -121,12 +121,18 @@ enum {
 #define HSCIF_SRE	BIT(15)	/* Sampling Rate Register Enable */
 
 /* SCPCR (Serial Port Control Register), SCIFA/SCIFB only */
-#define SCPCR_RTSC	BIT(4)	/* Serial Port RTS Pin / Output Pin */
-#define SCPCR_CTSC	BIT(3)	/* Serial Port CTS Pin / Input Pin */
+#define SCPCR_RTSC	BIT(4)	/* Serial Port RTS# Pin / Output Pin */
+#define SCPCR_CTSC	BIT(3)	/* Serial Port CTS# Pin / Input Pin */
+#define SCPCR_SCKC	BIT(2)	/* Serial Port SCK Pin / Output Pin */
+#define SCPCR_RXDC	BIT(1)	/* Serial Port RXD Pin / Input Pin */
+#define SCPCR_TXDC	BIT(0)	/* Serial Port TXD Pin / Output Pin */
 
 /* SCPDR (Serial Port Data Register), SCIFA/SCIFB only */
-#define SCPDR_RTSD	BIT(4)	/* Serial Port RTS Output Pin Data */
-#define SCPDR_CTSD	BIT(3)	/* Serial Port CTS Input Pin Data */
+#define SCPDR_RTSD	BIT(4)	/* Serial Port RTS# Output Pin Data */
+#define SCPDR_CTSD	BIT(3)	/* Serial Port CTS# Input Pin Data */
+#define SCPDR_SCKD	BIT(2)	/* Serial Port SCK Output Pin Data */
+#define SCPDR_RXDD	BIT(1)	/* Serial Port RXD Input Pin Data */
+#define SCPDR_TXDD	BIT(0)	/* Serial Port TXD Output Pin Data */
 
 /*
  * BRG Clock Select Register (Some SCIF and HSCIF)