[SPARC]: Fix initialization of sun4d SBUS interrupts.

1) Explicitly traverse to the root looking for the "sbi".
2) Grab the "board#" property from the sbi's parent and
   verify that this parent is an "io-unit" node.
3) Skip IRQ initialization when device lacks "reg" property.

Signed-off-by: David S. Miller <davem@davemloft.net>