of: Fix missing memory initialization on FDT unflattening

Any calls to dt_alloc() need to be zeroed. This is a temporary fix, but
the allocation function itself needs to zero memory before returning
it. This is a follow up to patch 9e401275, "of: fdt: fix memory
initialization for expanded DT" which fixed one call site but missed
another.

Signed-off-by: Grant Likely <grant.likely@linaro.org>
Acked-by: Wladislav Wiebe <wladislav.kw@gmail.com>
Cc: stable@vger.kernel.org