of: net: move of_net under net/

Rob suggests to move of_net.c from under drivers/of/ somewhere
to the networking code.

Suggested-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>