include: configs: soc64: Use CONFIG_SPL_ATF to differentiate bootfile

include: configs: soc64: Use CONFIG_SPL_ATF to differentiate bootfile

ATF boot flow (SPL->ATF->U-Boot Proper->OS) boot to OS via kernel.itb file
using bootm command.

Change to use CONFIG_SPL_ATF to differentiate the bootfile of default
environment variable. We shouldn't use CONFIG_FIT because it is enabled
by default for U-Boot Proper.

Signed-off-by: Tingting Meng <tingting.meng@altera.com>
Signed-off-by: Alif Zakuan Yuslaimi <alif.zakuan.yuslaimi@altera.com>
Reviewed-by: Tien Fong Chee <tien.fong.chee@altera.com>