Kconfig: Enable building of u-boot.itb on Rockchip platform

For all the Rockchip SoCs with SPL_LOAD_FIT enable, we need to build
u-boot.itb with U-Boot proper and ATF/OPTEE.

All the Rockchip boards with SPL_LOAD_FIT now supports FIT generator to
get ATF/OPTEE binary path from environment and pass the build even if no
ATF/OPTEE binary exist, so we can enable this feature for the rockchip
platform.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>