Register fields should support write-once and read write-once

[phausler]

For common definitions of register interfaces (such as SVD) there are two additional forms:

write-once: Read operations have an undefined results. Only the first write after reset has an effect.

read write-once: Read access is always permitted. Only the first write access after a reset will have an effect on the content. Other write operations have an undefined result.

[rauhul]

WriteOnce is something I have spent a quite a bit of time thinking about, but haven't been able find great solutions for.