hash: use 32-bit elements rings to save memory

The freelist and external bucket indices are 32b. Using rings
that use 32b element sizes will save memory.

Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Gavin Hu <gavin.hu@arm.com>
Reviewed-by: Ola Liljedahl <ola.liljedahl@arm.com>
Acked-by: Yipeng Wang <yipeng1.wang@intel.com>