ARM: dts: imx6qp: Specify imx6qp version of PCIe core

I.MX6Quad Plus has a slightly different version of PCIe core than reqular
i.MX6Quad.

Tested-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>