gpio: stp-xway: Implement get callback

Add an implementation to get the current GPIO state.

The callback is used by the leds-gpio driver for example, in case the
current LED/GPIO state should be kept during driver load.

Signed-off-by: Mathias Kresin <dev@kresin.me>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>