gpio: of: Check for "spi-cs-high" in child instead of parent node

"spi-cs-high" is going to be specified in child node of an SPI
controller's representing attached SPI device, so change the code to
look for it there, instead of checking parent node.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Cc: Chris Healy <cphealy@gmail.com>
Cc: linux-gpio@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>