This Answer Record covers how to quickly test u-boot over JTAG to see if it can program a QSPI flash which is marked as known to work in (Xilinx Answer 65463) but not yet supported by XSDK and Vivado.
1) Build the FSBL for A53-0 targeting your own board.
2) Use the ATF and u-boot.elf pre-built from the latest released image on the wiki:
3) Build a bootable image BOOT.bin.
4) Use the below script (it might require modification to point to your files) to download and start u-boot and download BOOT.bin to DDR (all via JTAG).
5) At the u-boot console, input the following commands:
sf probe 0 0 0
sf erase 0 size_of_BOOT.bin
sf write 0x8000 0 size_of_BOOT.bin