Update AllWinner 64 bit devices
    
    The AllWinner 64 bit devices originally had a SPL (sunxi-spl.bin) and
    a separate U-Boot (u-boot.itb) which we wrote out in two steps. There
    is now a combined option with both the U-Boot FIT and SPL combined
    into a single file like the 32 bit AllWinner devices so lets use that.
    
    Signed-off-by: Peter Robinson <pbrobinson@gmail.com>