rockchip: make_fit_atf.py: allow inclusion of a tee binary

A trusted execution environment should also get loaded as loadable from
a fit image, so add the possibility to present a tee.elf to make_fit_atf.py
that then gets included as additional loadable into the generated its.

For ease of integration the additional loadable is created as atf_(x+1)
after all others to re-use core generation loops.

Tested against the combinations of 1-part-atf and multi-part-atf each
time with and without a tee binary present.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>