arm: socfpga: stratix10: Add CONFIG_OF_EMBED

The dtb should be embedded in the u-boot-spl image so that
the CONFIG_SPL_TARGET of spl/u-boot-spl.hex includes it.

This also affects the main u-boot image, so adjust
CONFIG_SPL_FS_LOAD_PAYLOAD_NAME to u-boot.img which now
also includes the dtb.

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