backlight: sky81452: Convert to GPIO descriptors

The SKY81452 backlight driver just obtains a GPIO (named "gpios"
in the device tree) drives it high and leaves it high until the
driver is removed.

Switch to use GPIO descriptors for this, simple and
straight-forward.

Cc: Gyungoh Yoo <jack.yoo@skyworksinc.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>