dmcontrol.hartreset is WARL, not R/W (#490)

The spec says about the dmcontrol.hartreset field:
 "If this feature is not implemented, the bit always
stays 0, so after writing 1 the debugger can read
the register back to see if the feature is supported."

That makes it a WARL field not a R/W field. Changing the annotation to
make it more obvious to the reader.