gpio: raspberrypi-exp: mark OF related data as maybe unused

The driver can be compile tested with !CONFIG_OF making certain data
unused:

  drivers/gpio/gpio-raspberrypi-exp.c:237:34: error: ‘rpi_exp_gpio_ids’ defined but not used [-Werror=unused-const-variable=]
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

drivers/gpio/gpio-raspberrypi-exp.c

@@ -234,7 +234,7 @@ static int rpi_exp_gpio_probe(struct platform_device *pdev)
 	return devm_gpiochip_add_data(dev, &rpi_gpio->gc, rpi_gpio);
 }
 
-static const struct of_device_id rpi_exp_gpio_ids[] = {
+static const struct of_device_id rpi_exp_gpio_ids[] __maybe_unused = {
 	{ .compatible = "raspberrypi,firmware-gpio" },
 	{ }
 };