When searching the system for interrupt controllers, skip over any
that are explicitly disabled.

This makes interrupt controllers consistent with regular devices,
which can be marked as do-not-probe via the status = "disabled" dts
property.
Signed-off-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>
Acked-by: NMichal Simek <michal.simek@xilinx.com>
Signed-off-by: NRob Herring <rob.herring@calxeda.com>