net: marvell: prestera: Add cleanup of allocated fib_nodes

Do explicity cleanup on router_hw_fini, to ensure, that all allocated
objects cleaned. This will be used in cases,
when upper layer (cache) is not mapped to router_hw layer.

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: Jakub Kicinski <kuba@kernel.org>