spidev: Add "spidev" compatible string to silence warning

See: https://github.com/raspberrypi/linux/issues/1054

drivers/spi/spidev.c

@@ -696,6 +696,7 @@ static struct class *spidev_class;
 static const struct of_device_id spidev_dt_ids[] = {
 	{ .compatible = "rohm,dh2228fv" },
 	{ .compatible = "lineartechnology,ltc2488" },
+	{ .compatible = "spidev" },
 	{},
 };
 MODULE_DEVICE_TABLE(of, spidev_dt_ids);