uboot-phytec

rockchip: Remove obsolete references to pyelftools

make_fit_atf.py no longer requires pyelftools, and nothing else in the
rockchip build requires it either, so remove references to installing it
from the documentation.

Signed-off-by: Chris Webb <chris@arachsys.com>
Reviewed-by: Kever Yang <Kever.yang@rock-chips.com>