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>