Add arch_do_unload hook for arch-specific cleanup

Provide a new `arch_do_unload` hook that allows architectures to perform
cleanup actions during kexec unload. On ppc64, this hook resets the boot
CPU.

Signed-off-by: Shivang Upadhyay <shivangu@linux.ibm.com>
Signed-off-by: Simon Horman <horms@kernel.org>