of/spi: Support specifying chip select as active high via device tree

The patch allows to specify that an SPI device needs an active high chip
select.
Signed-off-by: Wolfgang Ocker <weo@reccoware.de>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>

Documentation/powerpc/booting-without-of.txt

@@ -1917,6 +1917,8 @@ platforms are moved over to use the flattened-device-tree model.
 			inverse clock polarity (CPOL) mode
     - spi-cpha        - (optional) Empty property indicating device requires
 			shifted clock phase (CPHA) mode
+    - spi-cs-high     - (optional) Empty property indicating device requires
+			chip select active high
 
     SPI example for an MPC5200 SPI bus:
 		spi@f00 {

drivers/of/of_spi.c

@@ -61,6 +61,8 @@ void of_register_spi_devices(struct spi_master *master, struct device_node *np)
 			spi->mode |= SPI_CPHA;
 		if (of_find_property(nc, "spi-cpol", NULL))
 			spi->mode |= SPI_CPOL;
+		if (of_find_property(nc, "spi-cs-high", NULL))
+			spi->mode |= SPI_CS_HIGH;
 
 		/* Device speed */
 		prop = of_get_property(nc, "spi-max-frequency", &len);