doc: mmc man-page

Provide a man-pages for the mmc command.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 5faf279..d330b14 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -32,6 +32,7 @@
    load
    loady
    mbr
+   mmc
    md
    pstore
    qfw