spi: gpio: clarify gpio chipselect language

Commit 74317984 (of_spi: add generic binding support to specify cs gpio)
introduced generic binding for gpio chip-select. The cs_gpio struct field,
however, is an internal implementation detail of the Linux SPI subsystem, and
should not be mentioned in the device tree binding documentation. Mention the
previously defined cs-gpios master node property instead.
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Mark Brown <broonie@linaro.org>

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

@@ -67,7 +67,7 @@ only 1(SINGLE), 2(DUAL) and 4(QUAD).
 Dual/Quad mode is not allowed when 3-wire mode is used.
 
 If a gpio chipselect is used for the SPI slave the gpio number will be passed
-via the cs_gpio
+via the SPI master node cs-gpios property.
 
 SPI example for an MPC5200 SPI bus:
 	spi@f00 {