dm: Tidy up some header file comments

Fix up the style of a few comments and add/clarify a few others.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/dm/uclass-id.h b/include/dm/uclass-id.h
index f0e691c..77ff9ea 100644
--- a/include/dm/uclass-id.h
+++ b/include/dm/uclass-id.h
@@ -19,7 +19,7 @@
 	UCLASS_TEST_FDT,
 
 	/* U-Boot uclasses start here */
-	UCLASS_GPIO,
+	UCLASS_GPIO,		/* Bank of general-purpose I/O pins */
 
 	UCLASS_COUNT,
 	UCLASS_INVALID = -1,