MFC r330365, r330371: OFW changes

r330365:
Move "common" Open Firmware parts of the loader used only on PowerPC to
the powerpc/ subdirectory. These have never used by SPARC and we have
no other (and almost certainly will have no other) Open Firmware platforms.
This makes the directory structure simpler and lets us avoid some
cargo-cult MI patterns on code that is, and always was,
architecture-specific.

r330371:
Where we can, pass the kernel an FDT facsimile of the OF device tree rather
than a pointer to Open Firmware by default. This eliminates a number of
potentially unsafe calls to firmware from the kernel and provides better
performance.

This feature is meant to be expanded until it is on by default
unconditionally and, ideally, we can then garbage-collect the
nightmare pile of hacks required to call into Open Firmware from a live
kernel.