csr: add we signal to CSR, CSRStatus

csr: add we signal to CSR, CSRStatus

Doing actions on register read is generally not a good design practice (it's
better to do separate register write to trigger actions) but in some very
specific cases being able to know that register has been read can solve cases
that are difficult to do with the recommended practives and that can justify
doing an exception.

This commit add a we signal to CSR, CSRStatus and this allow the logic to know
when the CSR, CSRStatus is read.