[POWERPC] Workaround oldworld OF bug with IRQs & P2P bridges

On some oldworld PowerMacs, OF doesn't assign interrupts properly
beyond P2P bridges. Fortunately, the fix is easy as all those machines
just wire all IRQ lines together to one IRQ which is assigned to the
bridge itself. We already have a special function for parsing Apple
OldWorld interrupts which are special, so let's add to it the ability
to walk up the PCI tree to find interrupts.

This fixes irqs on the lower slots of s900 clones among others.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>