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>
diff --git a/configs/ae350_rv64_xip_defconfig b/configs/ae350_rv64_xip_defconfig
index f7f2925..28afd81 100644
--- a/configs/ae350_rv64_xip_defconfig
+++ b/configs/ae350_rv64_xip_defconfig
@@ -16,7 +16,7 @@
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_BOOTP_PREFER_SERVERIP=y
 CONFIG_CMD_CACHE=y
-CONFIG_OF_BOARD=y
+CONFIG_OF_SEPARATE=y
 CONFIG_DEFAULT_DEVICE_TREE="ae350_64"
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_NET_RANDOM_ETHADDR=y