riscv: configs: AE350 will use CONFIG_OF_SEPARATE when boots from flash

When AE350 boots from flash, use CONFIG_OF_SEPARATE instead of
CONFIG_OF_BOARD.

Also remove unused code about prior_stage_fdt_address.
And modify CONFIG_SYS_FDT_BASE as flash address.

Signed-off-by: Rick Chen <rick@andestech.com>
Cc: Greentime Hu <greentime@andestech.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>

board/AndesTech/ax25-ae350/ax25-ae350.c

@@ -67,10 +67,6 @@ ulong board_flash_get_legacy(ulong base, int banknum, flash_info_t *info)
 
 void *board_fdt_blob_setup(void)
 {
-	void **ptr = (void *)&prior_stage_fdt_address;
-	if (fdt_magic(*ptr) == FDT_MAGIC)
-			return (void *)*ptr;
-
 	return (void *)CONFIG_SYS_FDT_BASE;
 }