[POWERPC] Fix CHRP platforms with only 8259

On CHRP platforms with only a 8259 controller, we should set the
default IRQ host to the 8259 driver's one for the IRQ probing
fallbacks to work in case the IRQ tree is incorrect (like on
Pegasos for example). Without this fix, we get a bunch of WARN_ON's
during boot.

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