[POWERPC] Make secondary CPUs call into kdump on reset exception

In the case of a system hang, the user will invoke soft-reset to
initiate the kdump boot.  If xmon is enabled, the CPU(s) enter into the
xmon debugger.   Unfortunately, the secondary CPU(s) will return to the
hung state when they exit from the debugger (returned from die() ->
system_reset_exception()).  This causes a problem in kdump since the
hung CPU(s) will not respond to the IPI sent from kdump.  This patch
fixes the issue by calling crash_kexec_secondary() directly from
system_reset_exception() without returning to the previous state.  These
secondary CPUs wait 5ms until the kdump boot is started by the primary
CPU.   In the case we exited from the debugger to "recover" (command 'x'
in xmon) the primary and the secondary CPUs will all return from die()
-> system_reset_exception() ->crash_kexec_secondary() wait 5ms, then
return to the previous state.  A kdump boot is not started in this case.

Signed-off-by: Haren Myneni <haren@us.ibm.com>
Signed-off-by: David Wilder <dwilder@us.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>