of/irq: Mark initialised interrupt controllers as populated

For interrupt controllers successfully initialised early via device-tree,
mark these interrupt controllers as populated so we don't unnecessarily
create a device and populate any platform data later on in the boot
sequence when we populate all the various platform devices.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Rob Herring <robh@kernel.org>