rockchip: doc: add rkmtd.rst

Add documention for Rockchip rkmtd virtual block device.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/board/rockchip/index.rst b/doc/board/rockchip/index.rst
index 0c377e9..9a87a03 100644
--- a/doc/board/rockchip/index.rst
+++ b/doc/board/rockchip/index.rst
@@ -8,3 +8,4 @@
    :maxdepth: 2
 
    rockchip
+   rkmtd