doc: man-page for blkcache

Provide a man-page for the blkcache command.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 53e4aee..9284d15 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -23,6 +23,7 @@
    cmd/addrmap
    cmd/askenv
    cmd/base
+   cmd/blkcache
    cmd/bootd
    cmd/bootdev
    cmd/bootefi