net: ipv6: Add Neighbor Discovery Protocol (NDP)

Implement basic of NDP. It doesn't include such things as Router
Solicitation, Router Advertisement and Redirect. It just has Neighbor
Solicitation and Neighbor Advertisement. Only these two features are used
in u-boot IPv6. Implementation of some NDP functions uses API that was
exposed in "net: ipv6: Add IPv6 basic primitives".

Also this patch inlcudes update in Makefile to build NDP.

Series-changes: 3
- Added structures and functions descriptions
- Fixed style problems

Series-changes: 4
- Fixed structures and functions description style

Signed-off-by: Viacheslav Mitrofanov <v.v.mitrofanov@yadro.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>