irqchip/irq-imx-gpcv2: Clear OF_POPULATED flag

Clear OF_POPULATED flag, so that GPC power domain driver[1] can be
bound to "gpc" node as well.

[1] https://lkml.org/lkml/2017/3/28/835



Cc: yurovsky@gmail.com
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>