arm: socfpga: stratix10: add CONFIG_SPL_TARGET

Stratix10 combines the u-boot-spl image into the fpga configuration
bitstream so that the SDM can load the processors memory.  This
process requires a hex format of the u-boot-spl image.
CONFIG_SPL_TARGET is set to "spl/u-boot-spl.hex"

Signed-off-by: Dalon Westergreen <dwesterg@gmail.com>