[pwrmgr] Alter behavior of ast clock enables

- instead of changing the clock enable values only in specific states,
  continuously evaluate the clock enable to ensure that a glitch cannot
  permanently disable a clock.

- this is done by outputting an "active" signal from the appropriate sparse
  state. Using this signal, the logic then decides whether to evaluate
  the clock enable on every cycle.  So if the clock enable were glitched "off",
  it would be restored on the next cycle before the clock enable is re-evaluated
  to be "on".  To completely disable a clock when not intended, the state itself
  must be altered, and that should be significantly more difficult due to the
  state being sparsely encoded.

- This addresses an issue similar to #11156

Signed-off-by: Timothy Chen <timothytim@google.com>