ARM: socfpga: update CONFIG_SPL_FS_LOAD_PAYLOAD_NAME to u-boot.img

Bring cyclone5 / arria5 / arria10 in line with convention and use
u-boot.img as CONFIG_SPL_FS_LOAD_PAYLOAD_NAME.

Signed-off-by: Dalon Westergreen <dalon.westergreen@intel.com>