dm: cache: Create a uclass for cache

The cache UCLASS will be used for configure settings that can be found
in a CPU's L2 cache controller.

Add a uclass and a test for cache.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>

include/dm/uclass-id.h

@@ -34,6 +34,7 @@ enum uclass_id {
 	UCLASS_BLK,		/* Block device */
 	UCLASS_BOARD,		/* Device information from hardware */
 	UCLASS_BOOTCOUNT,       /* Bootcount backing store */
+	UCLASS_CACHE,		/* Cache controller */
 	UCLASS_CLK,		/* Clock source, e.g. used by peripherals */
 	UCLASS_CPU,		/* CPU, typically part of an SoC */
 	UCLASS_CROS_EC,		/* Chrome OS EC */