ring: refactor exported headers

ring: refactor exported headers

For legacy modes, rename ring_generic/c11 to ring_generic/c11_pvt.
Furthermore, add new file ring_elem_pvt.h which includes ring_do_eq/deq
and ring element copy/delete APIs.

The update_tail internal helper has been prefixed with the library prefix.

For other modes, rename xx_c11_mem to xx_elem_pvt. Move all private APIs
into these new header files.

Finally, the external APIs and internal APIs will be separated from each
other. This can remind users not to use internal APIs and make ring
library easier to maintain.

Suggested-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Signed-off-by: Feifei Wang <feifei.wang2@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>