fpga: xiilnx-spi: Address warning about unused variable

warning: ‘xlnx_spi_of_match’ defined but not used
 [-Wunused-const-variable]
  static const struct of_device_id xlnx_spi_of_match[] = {

Fixes: 061c97d1 ("fpga manager: Add Xilinx slave serial SPI driver")
Cc: Tom Rix <trix@redhat.com>
Signed-off-by: Moritz Fischer <mdf@kernel.org>

drivers/fpga/xilinx-spi.c

@@ -256,11 +256,13 @@ static int xilinx_spi_probe(struct spi_device *spi)
 	return devm_fpga_mgr_register(&spi->dev, mgr);
 }
 
+#ifdef CONFIG_OF
 static const struct of_device_id xlnx_spi_of_match[] = {
 	{ .compatible = "xlnx,fpga-slave-serial", },
 	{}
 };
 MODULE_DEVICE_TABLE(of, xlnx_spi_of_match);
+#endif
 
 static struct spi_driver xilinx_slave_spi_driver = {
 	.driver = {