riscv: configs: AE350 will use CONFIG_OF_PRIOR_STAGE when boots from ram

riscv: configs: AE350 will use CONFIG_OF_PRIOR_STAGE when boots from ram

When AE350 boots from ram, use CONFIG_OF_PRIOR_STAGE instead
of CONFIG_OF_BOARD.

Signed-off-by: Rick Chen <[email protected]>
Cc: Greentime Hu <[email protected]>
Reviewed-by: Bin Meng <[email protected]>
Reviewed-by: Lukas Auer <[email protected]>