spi: tegra20-slink: 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-tegra20-slink.c

@@ -1001,7 +1001,7 @@ static const struct tegra_slink_chip_data tegra20_spi_cdata = {
 	.cs_hold_time = false,
 };
 
-static struct of_device_id tegra_slink_of_match[] = {
+static const struct of_device_id tegra_slink_of_match[] = {
 	{ .compatible = "nvidia,tegra30-slink", .data = &tegra30_spi_cdata, },
 	{ .compatible = "nvidia,tegra20-slink", .data = &tegra20_spi_cdata, },
 	{}