Add support for allocating irqs for bootbus devices

Some devices that can generate interrupts are connected directly to the
CPU through the bootbus on sun4d. This patch allows IRQs to be allocated
for such devices. The information used for allocating interrupts for
sbus devices are present at the corresponding SBI node. For bootbus
devices this information is present in the bootbus node.

Signed-off-by: Kjetil Oftedal <oftedal@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>