[PowerPC] PowerMac timebase sync for G4

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