The devicetree spec says in section 2.4:

The devicetree spec says in section 2.4:

  If an interrupt-generating device does not have an interrupt-parent
  property, its interrupt parent is assumed to be its devicetree
  parent.

Implement this aspect of the spec in the same way as Linux does by not
just looking at the immediate parent but keep on walking up the tree
until we hit a node with an "interrupt-controller" property or a node
that has an "interrupt-parent" property.

ok patrick@