dm: spi: Add soft_spi implementation

Add a new implementation of soft_spi that uses device tree to specify the
GPIOs. This will replace soft_spi_legacy for boards which use driver model.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>

doc/device-tree-bindings/spi/soft-spi.txt

@@ -0,0 +1,34 @@
+Soft SPI
+
+The soft SPI bus implementation allows the use of GPIO pins to simulate a
+SPI bus. No SPI host is required for this to work. The down-side is that the
+performance will typically be much lower than a real SPI bus.
+
+The soft SPI node requires the following properties:
+
+compatible: "u-boot,soft-spi"
+soft_spi_cs: GPIO number to use for SPI chip select (output)
+soft_spi_sclk: GPIO number to use for SPI clock (output)
+soft_spi_mosi: GPIO number to use for SPI MOSI line (output)
+soft_spi_miso GPIO number to use for SPI MISO line (input)
+spi-delay-us: Number of microseconds of delay between each CS transition
+
+The GPIOs should be specified as required by the GPIO controller referenced.
+The first cell holds the phandle of the controller and the second cell
+typically holds the GPIO number.
+
+
+Example:
+
+	soft-spi {
+		compatible = "u-boot,soft-spi";
+		cs-gpio = <&gpio 235 0>;	/* Y43 */
+		sclk-gpio = <&gpio 225 0>;	/* Y31 */
+		mosi-gpio = <&gpio 227 0>;	/* Y33 */
+		miso-gpio = <&gpio 224 0>;	/* Y30 */
+		spi-delay-us = <1>;
+		#address-cells = <1>;
+		#size-cells = <0>;
+		cs@0 {
+		};
+	};