pseries: Use the same interrupt swizzling for host bridges as p2p bridges

Currently the pseries PCI code uses a somewhat strange scheme of PCI irq
allocation - one per slot up to a maximum that's greater than the usual 4.
This scheme more or less worked, because we were able to tell the guest the
irq mapping in the device tree, however it's a bit odd and may break
assumptions in the future.  Worse, the array used to construct the dev
tree interrupt map was mis-sized, we got away with it only because it
happened that our SPAPR_PCI_NUM_LSI value was greater than 7.

This patch changes the pseries PCI code to use the same interrupt swizzling
scheme as is standardized for PCI to PCI bridges.  This makes for better
consistency, deals better with any devices which use multiple interrupt
pins and will make life easier in the future when we add passthrough of
what may be either a host bridge or a PCI to PCI bridge.  This won't break
existing guests, because they don't assume a particular mapping scheme for
host bridges, but just follow what we tell them in the device tree (also
updated to match, of course).  This patch also fixes the allocation of the
irq map.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>

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