xsecure update

Created by: silabs-krdosvik

This branch name is weird and not descriptive. The prupose of this PR is to update the xsecure assertion and vplan according to the SPEC.

The direct test passes if we do this !!temporary!! fix: (The ISS reads out what is actually the secureseed* CSRs' values, while the core reads out 0).

Some LFSR assertion passes only if the LFSR defalutseed is set to something else than zero.

ci_check passes, except from the debug test.

Feel free to comment on my English errors :-)

(I originally made two PRs in my own fork. The purpose of the first one was to split the xsecure module into several files, the purpose of the second one was to add the changes, so it can easily be compared with the old version. However, the PRs didnt seem to give that effect (I have propably done something wrong...). Let me know if you want me to give it another try, otherwise Ill just make one PR here.)