powerpc: Get merged kernel to compile and run on 32-bit SMP powermac.

This updates the powermac SMP code to use the mpic driver instead of
the openpic driver and fixes the SMP-dependent context switch code.
We had a subtle bug where we were using interrupt numbers 256-259 for
IPIs, but ppc32 had NR_IRQS = 256.  Moved the IPIs down to use interrupt
numbers 252-255 instead.

Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/sysdev/mpic.c

@@ -44,6 +44,9 @@ static struct mpic *mpics;
 static struct mpic *mpic_primary;
 static DEFINE_SPINLOCK(mpic_lock);
 
+#ifdef CONFIG_PPC32	/* XXX for now */
+#define distribute_irqs	CONFIG_IRQ_ALL_CPUS
+#endif
 
 /*
  * Register accessor functions