of/irq: Fix pSeries boot failure

of_irq_parse_raw() needs to return the correct interrupt controller
node when an interrupt-map property doesn't exist.

It allows of_irq_parse_raw() to return the node pointer of the interrupt
controller, rather than the parent bus. This allows ics_rtas_host_match()
to detect that the controller is a legacy 8259 and avoid using xics.
This avoids an RTAS assertion/crash during early kernel bootstrapping.

Signed-off-by: Jeremy Linton <lintonrjeremy@gmail.com>
Reviewed-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Grant Likely <grant.likely@linaro.org>