rockchip: make_fit_atf.py: fix loadables property set error

Commit b238e4b0 ("rockchip: Cleanup of make_fit_atf.py.") set
firmware = "atf_1";
loadables = "uboot","atf_1","atf_2";

Actually it should be:
firmware = "atf_1";
loadables = "uboot","atf_2","atf_3";

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>