riscv: Avoid the reserved memory fixup if src and dst point to the same place

The copy of reserved memory node from source dtb to destination dtb
can be avoided if they point to the same place. This is useful when
OF_PRIOR_STAGE is used.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>