powerpc/kexec: Move kexec files into a dedicated subdir.

arch/powerpc/kernel/ contains 8 files dedicated to kexec.

Move them into a dedicated subdirectory.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
[mpe: Move to a/p/kexec, drop the 'machine' naming and use 'core' instead]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/afbef97ec6a978574a5cf91a4441000e0a9da42a.1572351221.git.christophe.leroy@c-s.fr