spi: rspi: Add support for multiple native chip selects

RSPI variants on some SuperH or R-Mobile SoCs support multiple native
chip selects. Add support for this by configuring the SSL Assert Signal
Setting.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200102133822.29346-6-geert+renesas@glider.beSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-rspi.c

@@ -159,7 +159,7 @@
 #define SPCMD_SPIMOD_DUAL	SPCMD_SPIMOD0
 #define SPCMD_SPIMOD_QUAD	SPCMD_SPIMOD1
 #define SPCMD_SPRW		0x0010	/* SPI Read/Write Access (Dual/Quad) */
-#define SPCMD_SSLA_MASK		0x0030	/* SSL Assert Signal Setting (RSPI) */
+#define SPCMD_SSLA(i)		((i) << 4)	/* SSL Assert Signal Setting */
 #define SPCMD_BRDV_MASK		0x000c	/* Bit Rate Division Setting */
 #define SPCMD_CPOL		0x0002	/* Clock Polarity Setting */
 #define SPCMD_CPHA		0x0001	/* Clock Phase Setting */
@@ -933,6 +933,9 @@ static int rspi_prepare_message(struct spi_controller *ctlr,
 	if (spi->mode & SPI_CPHA)
 		rspi->spcmd |= SPCMD_CPHA;
 
+	/* Configure slave signal to assert */
+	rspi->spcmd |= SPCMD_SSLA(spi->chip_select);
+
 	/* CMOS output mode and MOSI signal from previous transfer */
 	rspi->sppcr = 0;
 	if (spi->mode & SPI_LOOP)