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>