spi: soft_spi: Parse cs-gpios only if num-chipselects is not <0>

Some boards don't have chipselect lines for leds so cs-gpios is not
specified in the dts leading to probing error. Fix it by making
behavior similar to the one in Linux, parse num-chipselects and
if it is zero, ignore cs-gpios.

Signed-off-by: Michael Polyntsov <michael.polyntsov@iopsys.eu>
Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>
diff --git a/doc/device-tree-bindings/spi/soft-spi.txt b/doc/device-tree-bindings/spi/soft-spi.txt
index bdf7e86..77b01b2 100644
--- a/doc/device-tree-bindings/spi/soft-spi.txt
+++ b/doc/device-tree-bindings/spi/soft-spi.txt
@@ -8,14 +8,15 @@
 
 Mandatory properties:
 compatible: "spi-gpio"
-cs-gpios: GPIOs to use for SPI chip select (output)
+cs-gpios: GPIOs to use for SPI chip select (output), not required if num-chipselects = <0>
 sck-gpios: GPIO to use for SPI clock (output)
 And at least one of:
 mosi-gpios: GPIO to use for SPI MOSI line (output)
 miso-gpios: GPIO to use for SPI MISO line (input)
 
-Optional propertie:
+Optional properties:
 spi-delay-us: Number of microseconds of delay between each CS transition
+num-chipselects: Number of chipselect lines
 
 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