xilinx: zynqmp: Config non zero SYS_SPI_U_BOOT_OFFS

This variable is pointing to offset is qspi where u-boot image is placed.
In our case it is location of u-boot.itb file. Offset is the same as is
used by Xilinx Zynq SoC.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>