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: NChia-Wei, Wang <chiawei_wang@aspeedtech.com>

configs/evb-ast2500_defconfig

@@ -22,7 +22,6 @@ CONFIG_CMD_MMC=y
 CONFIG_CMD_DHCP=y
 CONFIG_CMD_MII=y
 CONFIG_CMD_PING=y
-CONFIG_OF_EMBED=y
 CONFIG_ENV_OVERWRITE=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_NET_RANDOM_ETHADDR=y