treewide: Use OF_REAL instead of !OF_PLATDATA

Now that we have a 'positive' Kconfig option, use this instead of the
negative one, which is harder to understand.

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/develop/driver-model/of-plat.rst

@@ -215,7 +215,7 @@ For example:
 
     static int mmc_of_to_plat(struct udevice *dev)
     {
-    #if !CONFIG_IS_ENABLED(OF_PLATDATA)
+    #if CONFIG_IS_ENABLED(OF_REAL)
             /* Decode the devicetree data */
             struct mmc_plat *plat = dev_get_plat(dev);
             const void *blob = gd->fdt_blob;