[POWERPC] Cell interrupt rework

This patch reworks the cell iic interrupt handling so that:

 - Node ID is back in the interrupt number (only one IRQ host is created
for all nodes). This allows interrupts from sources on another node to
be routed non-locally. This will allow possibly one day to fix maxcpus=1
or 2 and still get interrupts from devices on BE 1. (A bit more fixing
is needed for that) and it will allow us to implement actual affinity
control of external interrupts.

 - Added handling of the IO exceptions interrupts (badly named, but I
re-used the name initially used by STI). Those are the interrupts
exposed by IIC_ISR and IIC_IRR, such as the IOC translation exception,
performance monitor, etc... Those get their special numbers in the IRQ
number space and are internally implemented as a cascade on unit 0xe,
class 1 of each node.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>