Add prestera_router_hw.c. This file contains functions, which track HW
objects relations and links. This include implicity creation of objects,
that needed by requested one and implicity removing of objects, which
reference counter is became zero.

We need this layer, because kernel callbacks not always mapped to
creation of single HW object. So let it be two different layers - one
for subscribing and parsing kernel structures, and another
(prestera_router_hw.c) for HW objects relations tracking.

There is two types of objects on router_hw layer:
 - Explicit objects (rif_entry) : created by higher layer.
 - Implicit objects (vr) : created on demand by explicit objects.
Co-developed-by: Taras Chornyi <tchornyi@marvell.com>
Signed-off-by: Taras Chornyi <tchornyi@marvell.com>
Co-developed-by: Oleksandr Mazur <oleksandr.mazur@plvision.eu>
Signed-off-by: Oleksandr Mazur <oleksandr.mazur@plvision.eu>
Signed-off-by: Yevhen Orlov <yevhen.orlov@plvision.eu>
Signed-off-by: David S. Miller <davem@davemloft.net>