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: 338b86c9 ("doc: boards: Add rockchip documentation")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>