spi: sh-msiof: Document frame start sync pulse mode

Unused, but nice to have it documented.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://patch.msgid.link/3d3acaab4a4125106a0655d28c09c050341c5eeb.1747401908.git.geert+renesas@glider.be
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: geertu

drivers/spi/spi-sh-msiof.c

@@ -86,6 +86,7 @@ struct sh_msiof_spi_priv {
 /* SITMDR1 and SIRMDR1 */
 #define SIMDR1_TRMD		BIT(31)		/* Transfer Mode (1 = Master mode) */
 #define SIMDR1_SYNCMD		GENMASK(29, 28)	/* SYNC Mode */
+#define SIMDR1_SYNCMD_PULSE	0U		/*   Frame start sync pulse */
 #define SIMDR1_SYNCMD_SPI	2U		/*   Level mode/SPI */
 #define SIMDR1_SYNCMD_LR	3U		/*   L/R mode */
 #define SIMDR1_SYNCAC		BIT(25)		/* Sync Polarity (1 = Active-low) */