ARM: imx6: mark GPC node as not populated after irq init to probe pm domain driver

Since IRQCHIP_DECLARE now flags the GPC node as already populated, the
GPC power domain driver is never probed unless we clear the flag again.

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