imx6: fix pcie interrupt mapping

The interrupt mapping for legacy interrupts was backwards causing interrupts
to be mapped improperly behind a PCIe to PCI bridge (ie GW16082 mezzanine).

Signed-off-by: Tim Harvey <tharvey@gateworks.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38519 3c298f89-4303-0410-b956-a3cf2f4a3e73