dt-bindings: gpio: fairchild,74hc595: Document chip select vs. latch clock

dt-bindings: gpio: fairchild,74hc595: Document chip select vs. latch clock

From looking at the data sheets, it is not obvious that CS# and latch
clock can be treated at the same, but doing so works fine and saves the
hassle of (1) trying to specify a SPI device without CS, and (2) adding
another property to drive the latch clock[1].

[1]: https://lore.kernel.org/lkml/20241213-gpio74-v1-2-fa2c089caf41@posteo.net/

Signed-off-by: J. Neuschäfer <j.ne@posteo.net>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20241224-gpio74-v2-3-bbcf14183191@posteo.net
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>