[PARISC] Allow nested interrupts

Our prior mode of operation didn't allow nested interrupts
because it makes the interrupt code much simpler.  However,
nested interrupts are better for latency.

This code uses the EIEM register to simulate level interrupts
and thus achieve nesting.

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>

arch/parisc/kernel/smp.c

@@ -262,6 +262,9 @@ ipi_interrupt(int irq, void *dev_id, struct pt_regs *regs)
 					this_cpu, which);
 				return IRQ_NONE;
 			} /* Switch */
+		/* let in any pending interrupts */
+		local_irq_enable();
+		local_irq_disable();
 		} /* while (ops) */
 	}
 	return IRQ_HANDLED;