Rewrite much of the embedded controller driver.

* Always use polled mode.  The intr approach did not work for many
  controllers and required the hw.acpi.ec.event_driven workaround.
* Only use an edge (not level) triggered GPE handler
* Add sc->ec_mtx for locking operations to a single EC.  There were
  many race conditions earlier between an SCI event and EcRead/Write.
* Use 1 ms as the global lock timeout
* Only acquire global lock if _GLK != 0
* Update EcWaitEvent to use an incremental backoff delay in its
  poll loop.  Wait 50 ms max instead of 10.  Most ECs respond
  in < 5 us (50 us when heavily loaded).  However, some time out
  occasionally even with a 10 ms timeout.  For delays past 1 ms, use
  msleep instead of DELAY to give SCI interrupts a chance to occur.
* Add EcCommand to send a command and wait for the appropriate event.
* The hw.acpi.ec.event_driven tunable is no longer applicable and
  has been removed.

Ideas from:	Linux

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