doc: bindings: soft-spi: update documentation to match the code

Linux bindings have been introduced in the code (removing the U-Boot
specific ones) without documentation update. Compatible string has
changed, as well as the four GPIO properties. Reflect this by updating
the soft-spi.txt documentation.

Fixes: 102412c4 ("dm: spi: soft_spi: switch to use linux compatible string")
Signed-off-by: Miquel Raynal <miquel.raynal@free-electrons.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>

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

@@ -6,11 +6,15 @@ 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)
+Mandatory properties:
+compatible: "spi-gpio"
+cs-gpios: GPIOs to use for SPI chip select (output)
+gpio-sck: GPIO to use for SPI clock (output)
+And at least one of:
+gpio-mosi: GPIO to use for SPI MOSI line (output)
+gpio-miso: GPIO to use for SPI MISO line (input)
+
+Optional propertie:
 spi-delay-us: Number of microseconds of delay between each CS transition
 
 The GPIOs should be specified as required by the GPIO controller referenced.
@@ -21,11 +25,11 @@ 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 */
+		compatible = "spi-gpio";
+		cs-gpios = <&gpio 235 0>;	/* Y43 */
+		gpio-sck = <&gpio 225 0>;	/* Y31 */
+		gpio-mosi = <&gpio 227 0>;	/* Y33 */
+		gpio-miso = <&gpio 224 0>;	/* Y30 */
 		spi-delay-us = <1>;
 		#address-cells = <1>;
 		#size-cells = <0>;