spi: dw: introduce support of loopback mode

For testing purposes it's good to have a loopback mode enabled. The patch adds
necessary bits for that.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-dw.c

@@ -591,6 +591,9 @@ static int dw_spi_setup(struct spi_device *spi)
 			| (spi->mode  << SPI_MODE_OFFSET)
 			| (chip->tmode << SPI_TMOD_OFFSET);
 
+	if (spi->mode & SPI_LOOP)
+		chip->cr0 |= 1 << SPI_SRL_OFFSET;
+
 	if (gpio_is_valid(spi->cs_gpio)) {
 		ret = gpio_direction_output(spi->cs_gpio,
 				!(spi->mode & SPI_CS_HIGH));
@@ -652,7 +655,7 @@ int dw_spi_add_host(struct device *dev, struct dw_spi *dws)
 		goto err_free_master;
 	}
 
-	master->mode_bits = SPI_CPOL | SPI_CPHA;
+	master->mode_bits = SPI_CPOL | SPI_CPHA | SPI_LOOP;
 	master->bits_per_word_mask = SPI_BPW_MASK(8) | SPI_BPW_MASK(16);
 	master->bus_num = dws->bus_num;
 	master->num_chipselect = dws->num_cs;