Support multiple outstanding load operations to the Dcache

Created by: cfuguet

This modification introduces a pending load table that tracks outstanding load operations to the Dcache. The depth of this table is a parameter in the target configuration package.

The ID in the request from the load/store unit must be mirrored by the Dcache in the response. This allows to match a given response to its corresponding request. Responses can be given (by the Dcache) in a different order that the one of requests.

To be able to send one load request per cycle, the pending load table shall have at least 2 entries. This is to avoid a combinational path between the request and the response interfaces. This has a minor impact in the area anyway because each entry of the buffer has roughly 10 bits (FFs).