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/mtd/spi/sf-uclass.c b/drivers/mtd/spi/sf-uclass.c
index cfce00e..63d1629 100644
--- a/drivers/mtd/spi/sf-uclass.c
+++ b/drivers/mtd/spi/sf-uclass.c
@@ -3,6 +3,8 @@
  * Copyright (c) 2014 Google, Inc
  */
 
+#define LOG_CATEGORY UCLASS_SPI_FLASH
+
 #include <common.h>
 #include <dm.h>
 #include <log.h>