disk: Add an option for partitions in SPL

In some cases we do not want to enable partition support in SPL. Add an
option to allow this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/block/blk-uclass.c b/drivers/block/blk-uclass.c
index bee1cd6..f9f05f4 100644
--- a/drivers/block/blk-uclass.c
+++ b/drivers/block/blk-uclass.c
@@ -712,7 +712,7 @@
 
 static int blk_post_probe(struct udevice *dev)
 {
-	if (IS_ENABLED(CONFIG_PARTITIONS) &&
+	if (CONFIG_IS_ENABLED(PARTITIONS) &&
 	    IS_ENABLED(CONFIG_HAVE_BLOCK_DEVICE)) {
 		struct blk_desc *desc = dev_get_uclass_plat(dev);