rockchip: make_fit_atf: use correct fdt_x references in config nodes

rockchip: make_fit_atf: use correct fdt_x references in config nodes

The script iterates over the given devicetrees and creates both
fdt_x node as well as a conf-node for each passed dt.

But there is a slight bug in that it always references fdt_1 in each
conf node instead of the matching fdt_x as expected.

So fix that by referencing the number of the current dt similar to
how the fdt_x nodes gets created.

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