RISC-V: Make sure memblock reserves the memory containing DT

Currently, the memory containing DT is not reserved. Thus, that region
of memory can be reallocated or reused for other purposes. This may result
in  corrupted DT for nommu virt board in Qemu. We may not face any issue
in kendryte as DT is embedded in the kernel image for that.

Fixes: 6bd33e1e ("riscv: add nommu support")
Cc: stable@vger.kernel.org
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>