[POWERPC] Fix RTAS os-term usage on kernel panic

The rtas_os_term() routine was being called at the wrong time.
The actual rtas call "os-term" will not ever return, and so
calling it from the panic notifier is too early.  Instead,
call it from the machine_reset() call.

This splits the rtas_os_term() routine into two: one part to capture
the kernel panic message, invoked during the panic notifier, and
another part that is invoked during machine_reset().

Prior to this patch, the os-term call was never being made,
because panic_timeout was always non-zero.  Calling os-term
helps keep the hypervisor happy!  We have to keep the hypervisor
happy to avoid service, dump and error reporting problems.

Signed-off-by: Linas Vepstas <linas@austin.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>