ring: support configurable element size

ring: support configurable element size

Current APIs assume ring elements to be pointers. However, in many
use cases, the size can be different. Add new APIs to support
configurable ring element sizes.

Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Dharmik Thakkar <dharmik.thakkar@arm.com>
Reviewed-by: Gavin Hu <gavin.hu@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>