gpio: xra1403: mark OF related data as maybe unused

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

  drivers/gpio/gpio-xra1403.c:198:34: error: ‘xra1403_spi_of_match’ 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-xra1403.c

@@ -195,7 +195,7 @@ static const struct spi_device_id xra1403_ids[] = {
 };
 MODULE_DEVICE_TABLE(spi, xra1403_ids);
 
-static const struct of_device_id xra1403_spi_of_match[] = {
+static const struct of_device_id xra1403_spi_of_match[] __maybe_unused = {
 	{ .compatible = "exar,xra1403" },
 	{},
 };