ARM: dts: imx6: add PCIe interrupt mapping properties

As defined by the common PCI bindings.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>