of: irq: Ignore disabled intc's when searching map

When searching the interrupt map, if a matched parent is disabled, just
ignore it and move on with the search.

This allows for specifying connection of a single device IRQ to
multiple interrupt controllers via the interrupt map schema. This change
allows for selection of the active interrupt controller via the already
existing status = "disabled" mechanism.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>