ARM: imx6: correct i.MX6 PCIe interrupt routing

The PCIe interrupts are also routed through the GPC. This has been
missed from the conversion to stacked IRQ domains as the PCIe
controller uses an explicit interrupt map and thus doesn't inherit
the SoC global interrupt parent.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Cc: <stable@vger.kernel.org> # 4.1
Signed-off-by: Shawn Guo <shawnguo@kernel.org>