powerpc/kexec: Add to and tidy debug/comments in machine_kexec64.c

Tidies some typos, KERN_INFO-ise an info msg, and add a debug msg showing
when the final sequence starts.

Also adds a comment to kexec_prepare_cpus_wait() to make note of a possible
problem; the need for kexec to deal with CPUs that failed to originally start
up.

Signed-off-by: Matt Evans <matt@ozlabs.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>