[POWERPC] Copy over headers from arch/ppc to arch/powerpc that we need

To build arch/powerpc without including asm-ppc/ we need these files
in asm-powerpc/

Moved some headers under arch/powerpc/platforms if they were only used by
platform or driver files and fixed up the source file includes to match
the new locations

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>