[POWERPC] reboot when panic_timout is set

Only call into RTAS when booted with panic=0 because the RTAS call
does not return.  The system has to be rebooted via the HMC or via the
management console right now.  This is cumbersome and not what the
default panic=180 is supposed to do.

Signed-off-by: Olaf Hering <olh@suse.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>