riscv: Expand the DT size before copy reserved memory node

riscv: Expand the DT size before copy reserved memory node

The FDT blob might not have sufficient space to hold a copy of
reserved memory node. Expand it before the copy.

Reported-by: Rick Chen <[email protected]>
Signed-off-by: Bin Meng <[email protected]>
Reviewed-by: Atish Patra <[email protected]>
Reviewed-by: Rick Chen <[email protected]>