gpio: pisosr: Use devm_gpiod_get_optional for gpio->load_gpio

gpio->load_gpio is optional, so use devm_gpiod_get_optional instead.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>