ppc: Make uninorth interrupt swizzling identical to Grackle

It's currently broken as it uses an incorrect shift, it tries
to use the slot number but uses the top bits of the bus number
instead.

Note: Neither implementation matches what OpenBIOS ends up putting
in the device-tree either, which will have to be fixed separately.

This is not quite correct for modelling a real Mac since Apple
tend to tie all 4 interrupt lines of a slot together and have
separate interrupts for every slot and every motherboard devices
going straight to the PIC but we'll sort that out later.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>