Summary:
Disable timebase on (some) AIM platforms (tested on PowerMac G4) prior
to synchronization.
Some platforms use a GPIO to enable and disable timebase, while others
use a platform function.
This mirrors 0d69f00b on mpc85xx.
Todo:
* Implement various G5 timebase controls.
* Print out platform code on unknown G5s so we can collect it.
* Change API to be give/take pairs like Linux does so it's possible to
do a software sync protocol.
Reviewed By: #powerpc, jhibbits
Subscribers: mikael, markmi_dsl-only.net, luporl, alfredo
Tags: #powerpc
Differential Revision: https://reviews.freebsd.org/D29136
c583b02587