lib: zero-out memory allocated using sbi_scratch_alloc_offset()

We should zero-out memory allocated from extra scratch space using
sbi_scratch_alloc_offset() API hence this patch. This will not
impact performance because we mostly allocate from extra scratch
space only at cold boot time.

Signed-off-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Atish Patra <atish.patra@wdc.com>