atkbd: Reduce polling rate from 10Hz to ~1Hz.

In my understanding this is only needed to workaround lost interrupts.
I was thinking to remove it completely, but the comment about edge-
triggered interrupt may be true and needs deeper investigation. ~1Hz
should be often enough to handle the supposedly rare loss cases, but
rare enough to not appear in top. Add sysctl hw.atkbd.hz to tune it.

MFC after: 1 month