of: irq: Ignore disabled interrupt controllers

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