icss.core: allow read/write access to constant regs
icss.core: allow read/write access to constant regs

even when core is not halted (although it needs to be halted once to
initialize the cached constants).

Any attempt to modify constant registers apart from bits 8-11 of c24-c27
or bits 8-23 of c28-c31 will be rejected with a ValueError.

mvduin/py-uio,mvduin/py-uio,mvduin/py-uio,mvduin/py-uio,mvduin/py-uio