irqchip: Fix probing deferal when using IRQCHIP_PLATFORM_DRIVER helpers

When probing an interrupt controller that is behind a parent,
we try to check whether the parent domain is available as
an indication that we can actually try to probe.

Unfortunately, we are checking this with the firmware node of
the about to be probed device, not the parent. This is obviously
bound to fail.

Instead, use the parent node.

Fixes: f8410e62 ("irqchip: Add IRQCHIP_PLATFORM_DRIVER_BEGIN/END and IRQCHIP_MATCH helper macros")
Reported-by: John Stultz <john.stultz@linaro.org>
Tested-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>