riscv: Copy the reserved-memory nodes to final DT

The DT used by U-Boot may be different from the DT being passed to
the OS if the DT is loaded from external media such as network or
mmc. In that case, the reserved-memory node needs to be copied to
the DT passed to the OS.

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>