[POWERPC] Move some kexec logic into machine_kexec.c

We're missing a few functions for kexec to compile on 32-bit. There's
nothing really 64-bit specific about the 64-bit versions, so make them
generic rather than adding empty definitions for 32-bit.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>