[POWERPC] Make OF irq map code detect more error cases

Device-tree bugs on js20 with some versions of SLOF were causing the
interrupt for IDE to not be parsed correctly and fail to boot. This
patch adds a bit more sanity checking to the parser to detect some of
those errors and fail instead of returning bogus information.  The
powerpc PCI code can then trigger a fallback that works on those
machines.

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