Lock writable register

[lorenzschmid]

Setting the lock attribute to true, a writable register or field can be temporarily locked. While being locked, the register or field maintains its current value and ignores any write requests. Furthermore, while being locked, the output can also be altered to a predetermined value set via lock-value attribute. The locking behavior is controlled via an external signal (defined through the lock-port attribute on the memory map).

We require such a feature, when our logic is operating in a test mode, i.e. in scan test: Some registers control external logic, whose control we require to remain constant.

[lorenzschmid]

It just occurred to me that I have not added any tests yet. Will do so first.

[tgingold-cern]

ok!