As indicated by the question the solution represent the device as a system and therefore all documents that are describing the system as a whole shall be located under the solution.
For Example: If I wish to develop a new iPhone 6 then my solution will be called iPhone 6. All documents that describe the iPhone 6 as a phone such as main characteristics, size, weight etc. will be located there as well all other documents that will be describe next.
MY Device 1.0 (Solution)
  |-> Market Requirements
  |-> System Requirements
  |-> Product Requirements
  |-> System Test Description
  |-> …

Device Software 7.0 (Project)
  |-> 
  |->
  |->
Device Hardware 2.1 (Project)
  |-> 
  |-> 
  |->
Device Mechanical 3.0 (Project)
  |->
  |->
  |->
 
Component documents, they are not just a documents that describe the implementation of the system requirements but they are also documents that are used to manage the ALM process of each component. Therefore it necessary that the documents will have the correct development version of the component. To do that we must put the document in the same project and version where the component is being developed.
 
MY Device 1.0 (Solution)
  |-> Market Requirements
  |-> System Requirements
  |-> Product Requirements
  |-> System Test Description
  |-> …

Device Software 7.0 (Project)
  |-> Software Requirement Specification 7.0 (DOC)
  |-> Software Test Description 7.0 (DOC)
  |-> Software Detail Design 7.0 (DOC)
Device Hardware 2.1 (Project)
  |-> Hardware Requirement Specification 2.1 (DOC)
  |-> Hardware Test Description 2.1 (DOC)
  |-> Hardware Detail Design 2.1 (DOC)
Device Mechanical 3.0 (Project)
  |-> Mechanical Requirement Specification 3.0 (DOC)
  |-> Mechanical Detail Design 3.0 (DOC)
  |-> Mechanical Test Description 3.0 (DOC)