powerpc: Add NMI IPI infrastructure

Add a simple NMI IPI system that handles concurrency and reentrancy.

The platform does not have to implement a true non-maskable interrupt,
the default is to simply use the debugger break IPI message. This has
now been co-opted for a general IPI message, and users (debugger and
crash) have been reimplemented on top of the NMI system.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Incorporate incremental fixes from Nick]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/cell/interrupt.c

@@ -211,7 +211,7 @@ void iic_request_IPIs(void)
 	iic_request_ipi(PPC_MSG_CALL_FUNCTION);
 	iic_request_ipi(PPC_MSG_RESCHEDULE);
 	iic_request_ipi(PPC_MSG_TICK_BROADCAST);
-	iic_request_ipi(PPC_MSG_DEBUGGER_BREAK);
+	iic_request_ipi(PPC_MSG_NMI_IPI);
 }
 
 #endif /* CONFIG_SMP */