gpio: Cut old SX150X Kconfig option

The SX150X driver was moved over to pin control a while back.
The GPIO Kconfig symbol creates a circular dependency since
it requires GPIOLIB and the pin control driver selects GPIOLIB
so get rid of the old annoying Kconfig option.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>