rockchip: board: puma_rk3399: rename ATF firmware

prefix the bl31 firmware needed to build uboot.itb so it can coexist in
the build area with ATFs from other boards (i.e. lion_rk3368)

Signed-off-by: Klaus Goger <klaus.goger@theobroma-systems.com>
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>