arm: aspeed: Fix AST2600 SPL external DTB alignment

For AST2600 SPL with CONFIG_SPL_SEPARATE_BSS=y and
CONFIG_OF_SEPARATE=y, fdt_find_separate() expects the appended control
DTB at _image_binary_end.

The AST2600 SPL linker script aligned _image_binary_end via the
.binman_sym_table section. In builds where that section has no emitted
contents, objcopy -O binary does not include the alignment bytes in
spl/u-boot-spl-nodtb.bin. This causes the nodtb binary to end before
_image_binary_end, so the appended DTB is placed too early.

On evb-ast2600 this produced:

  sizeof(spl/u-boot-spl-nodtb.bin) = 0xc92c
  _image_binary_end                = 0xc930

SPL then looked four bytes past the FDT header and failed with:

  Missing DTB
  No serial driver found

Move the 8-byte alignment into the preceding emitted __u_boot_list
section so the objcopy output and _image_binary_end agree.

Signed-off-by: Quentin Strydom <qstrydom0@gmail.com>