spi/dt: DT bindings documentation: "num-cs" property for SPI controllers

Several SPI controller drivers have defined differently named properties for
the number of chip selects.  Now adding "num-cs" as a reference name for new
bindings.
Signed-off-by: NRoland Stigge <stigge@antcom.de>
Reviewed-by: NLinus Walleij <linus.walleij@linaro.org>
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>

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

@@ -21,6 +21,9 @@ assumption that board specific platform code will be used to manage
 chip selects.  Individual drivers can define additional properties to
 support describing the chip select layout.
 
+Optional property:
+- num-cs : total number of chipselects
+
 SPI slave nodes must be children of the SPI master node and can
 contain the following properties.
 - reg             - (required) chip select address of device.