dm: define LOG_CATEGORY for all uclass

Define LOG_CATEGORY for all uclass to allow filtering with
log command.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/button/button-uclass.c b/drivers/button/button-uclass.c
index e06d3eb..e33ed7d 100644
--- a/drivers/button/button-uclass.c
+++ b/drivers/button/button-uclass.c
@@ -5,6 +5,8 @@
  * Based on led-uclass.c
  */
 
+#define LOG_CATEGORY UCLASS_BUTTON
+
 #include <common.h>
 #include <button.h>
 #include <dm.h>