[PowerPC] PowerMac timebase sync for G4
Disable timebase on (some) AIM platforms (tested on PowerMac G4) prior
Some platforms use a GPIO to enable and disable timebase, while others
use a platform function.
This mirrors 0d69f00b on mpc85xx.
- 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.