configs: evb-ast2500: Convert to OF_SEPARATE

Switch DTB provider form OF_EMBED to OF_SEPARATE
to avoid the compile warning message:

  ==================== WARNING ======================
  CONFIG_OF_EMBED is enabled. This option should only
  be used for debugging purposes. Please use
  CONFIG_OF_SEPARATE for boards in mainline.
  See doc/README.fdt-control for more info.
  ====================================================

Signed-off-by: Chia-Wei, Wang <chiawei_wang@aspeedtech.com>