doc: board: add Rockchip to doc/board/index.rst

Fix a build error

checking consistency...
/doc/board/rockchip/index.rst:
WARNING: document isn't included in any toctree

Fixes: 338b86c9b305 ("doc: boards: Add rockchip documentation")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/board/index.rst b/doc/board/index.rst
index 00e72f5..b8b956d 100644
--- a/doc/board/index.rst
+++ b/doc/board/index.rst
@@ -14,5 +14,6 @@
    google/index
    intel/index
    renesas/index
+   rockchip/index
    sifive/index
    xilinx/index