of/irq: Mark interrupt controllers as populated before initialisation

That way the init callback may clear the flag again, in case of drivers
split between early irq chip and a normal platform driver.

Fixes: 15cc2ed6 ("of/irq: Mark initialised interrupt controllers as populated")
Suggested-by: Rob Herring <robh@kernel.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Rob Herring <robh@kernel.org>