spi: Add support for specifying 3-wire mode via device tree

This patch allows to specify that a SPI device is connected in 3-wire mode via
device tree.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

Documentation/devicetree/bindings/spi/spi-bus.txt

@@ -53,6 +53,8 @@ contain the following properties.
     		shifted clock phase (CPHA) mode
 - spi-cs-high     - (optional) Empty property indicating device requires
     		chip select active high
+- spi-3wire       - (optional) Empty property indicating device requires
+    		    3-wire mode.
 
 If a gpio chipselect is used for the SPI slave the gpio number will be passed
 via the cs_gpio

drivers/spi/spi.c

@@ -861,6 +861,8 @@ static void of_register_spi_devices(struct spi_master *master)
 			spi->mode |= SPI_CPOL;
 		if (of_find_property(nc, "spi-cs-high", NULL))
 			spi->mode |= SPI_CS_HIGH;
+		if (of_find_property(nc, "spi-3wire", NULL))
+			spi->mode |= SPI_3WIRE;
 
 		/* Device speed */
 		prop = of_get_property(nc, "spi-max-frequency", &len);