mtd: nand: mxc: Test CONFIG_OF instead of CONFIG_OF_MTD

We are about to drop the OF_MTD Kconfig option. Test CONFIG_OF
activation instead of CONFIG_OF_MTD.
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>

drivers/mtd/nand/mxc_nand.c

@@ -1565,7 +1565,7 @@ static const struct platform_device_id mxcnd_devtype[] = {
 };
 MODULE_DEVICE_TABLE(platform, mxcnd_devtype);
 
-#ifdef CONFIG_OF_MTD
+#ifdef CONFIG_OF
 static const struct of_device_id mxcnd_dt_ids[] = {
 	{
 		.compatible = "fsl,imx21-nand",