riscv: Setup reserved-memory node for FU540

FU540 uses OF_SEPARATE instead of OF_PRIOR_STAGE.

Enable OF_BOARD_FIXUP to update the DT with reserved-memory node.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>