U-boot-spl partition selection

Hi all,
I work on an update mechanism for our device. For that, I plan to create two partitions which each contains the FPGA firmware in a FIT image, the Linux kernel, the device tree and so on.
On of it is the “active” and one the “passive” partition.
I plan to select the right partition in the u-boot-spl and load the additional files from there.
Now my question is: Where can I configure the Partition number in u-boot-spl and also u-boot where the firmware is located?
I use the current u-boot version 2020.07 from https://github.com/altera-opensource/u-boot-socfpga

Thanks for any hint.