PCI: imx6: Add support for MSI

This patch adds support for Message Signaled Interrupts in the imx6-pcie
driver.

Signed-off-by: Harro Haan <hrhaan@gmail.com>
Signed-off-by: Juergen Beisert <jbe@pengutronix.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Richard Zhu <r65037@freescale.com>