doc: rockchip: Update and improve info on rk3308, TPL and TF-A

Update and improve documentation about build steps for SoCs that
require using TF-A and TPL binaries provided by Rockchip, such as
rk3308. Add rk3308 boards case to rST document. Add ROCK Pi S in
the list of supported boards. Minor page format improvements.

Signed-off-by: Massimo Pegorer <massimo.pegorer+oss@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
2 files changed