spi: spi-gpio: Make of_device_id array const

Make of_device_id array const, because all OF functions
handle it as const.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

drivers/spi/spi-gpio.c

@@ -340,7 +340,7 @@ done:
 }
 
 #ifdef CONFIG_OF
-static struct of_device_id spi_gpio_dt_ids[] = {
+static const struct of_device_id spi_gpio_dt_ids[] = {
 	{ .compatible = "spi-gpio" },
 	{}
 };