powerpc: Fix for Pegasos keyboard and mouse

[See http://lists.ozlabs.org/pipermail/linuxppc-dev/2010-October/086424.html


and followups. Part of the commit message is directly copied from that.]

Commit 540c6c39 tries to find i8042 IRQs in
the device-tree but doesn't fall back to the old hardcoded 1 and 12 in all
failure cases.

Specifically, the case where the device-tree contains nothing matching
pnpPNP,303 or pnpPNP,f03 doesn't seem to be handled well. It sort of falls
through to the old code, but leaves the IRQs set to 0.

Signed-off-by: Gabriel Paubert <paubert@iram.es>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>