powerpc/mpc85xx: Synchronize timebase the platform correct way

Summary:
To safely synchronize timebase we need to disable the timebase on all
cores, set timebase, and resynchronize. This adds two new devices, mutually
exclusive, which attach on the SoC simplebus, to freeze and unfreeze the
timebase. The devices are singletons, and platform-specific, so no reason
to make them optional and in separate files.

This was found to be necessary for top(1) to work correctly on an AmigaOne
X5000 (P5020 SoC). It also fixes bufdaemon and bufspacedaemon hangs at
shutdown.

Test Plan: Regression test on various Book-E hardware.

Reviewed by: nwhitehorn
Tested by: Brandon Bergren (git_bdragon.rtk0.net)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D19208