heathrow: change heathrow_pic_init() to return the heathrow device

heathrow: change heathrow_pic_init() to return the heathrow device

This enables the device to be made available during the setup of the Old World
machine. In order to pass back the previous set of IRQs we temporarily introduce
a new pic_irqs parameter until it can be removed.

An additional benefit of this change is that it is also possible to remove the
pic_mem pointer used for macio by accessing the memory region via sysbus.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>