powerpc/xmon: Wait for secondaries before IPI'ing on system reset

An externally triggered system reset (e.g., via QEMU nmi command, or pseries
reset button) can cause system reset interrupts on all CPUs. In case this causes
xmon to be entered, it is undesirable for the primary (first) CPU into xmon to
trigger an NMI IPI to others, because this may cause a nested system reset
interrupt.

So spin for a time waiting for secondaries to join xmon before performing the
NMI IPI, similarly to what the crash dump code does.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Only do it when we come in from system reset, not via sysrq etc.]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>