dax/sim: ad9910/ad9912 removed value constraints and other minor improvements

dax/sim: ad9910 simulation now includes more rounding errors, less input value constraints, few more cfr1 bits supported

removed some value constraints, closes #153 (closed)

additionally:

1. rounding errors more realistic
2. ad9910 more cfr1 support including power down bits and async phase clear (renames phase clear signal from "phase_autoclear" to "phase_clear".