lib: sbi_scratch: use bitwise ops in sbi_scratch_alloc_offset()

Instead of using loop to make "size" machine word aligned, we should
use bitwise ops.

Signed-off-by: Li Jinpei <leekingp1994@163.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>