cmd:Elaborate 'blkcache' cmd HELP statement

HELP description is provided for ‘configure’ sub-command
of ‘blkcache’.

Signed-off-by: Adarsh Babu Kalepalli <opensource.kab@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/blkcache.c b/cmd/blkcache.c
index 25f252e..47ea1ec 100644
--- a/cmd/blkcache.c
+++ b/cmd/blkcache.c
@@ -83,5 +83,6 @@
 	blkcache, 4, 0, do_blkcache,
 	"block cache diagnostics and control",
 	"show - show and reset statistics\n"
-	"blkcache configure blocks entries\n"
+	"blkcache configure <blocks> <entries> "
+	"- set max blocks per entry and max cache entries\n"
 );