spi: dt-bindings: clarify CS behavior for spi-cs-high and gpio descriptors

Behavior of CS signal in combination of spi-cs-high and gpio descriptors
is not clearly defined and documented. So clarify the documentation

Cc: linus.walleij@linaro.org
Cc: linux-gpio@vger.kernel.org
Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/3bed61807fff6268789e7d411412fbc5cd6ffe2a.1607507863.git.hns@goldelico.com


Signed-off-by: Mark Brown <broonie@kernel.org>