Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Introduce PLATFORMMETHOD_END and use it.

Factor out platform dependent things unrelated to device drivers into a
new platform module. These are probed in early boot, and have the
responsibility of determining the layout of physical memory, determining
the CPU timebase frequency, and handling the zoo of SMP mechanisms
found on PowerPC.

Reviewed by: marcel, raj
Book-E parts by: raj