powerpc: Merge asm-ppc/kexec.h and asm-ppc64/kexec.h

Merge include/asm-ppc/kexec.h and include/asm-ppc64/kexec.h.

The only thing that's really changed is that we now allocate crash_notes
properly on PPC32. It's address is exported via sysfs, so it's not correct
for it to be a pointer.

I've also removed some of the "we don't use this" comments, because they're
wrong (or perhaps were referring only to arch code).

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>