PCI: imx: Enable MSI from downstream components

The MSI Enable bit in the MSI Capability (PCIe r4.0, sec 7.7.1.2) controls
whether a Function can request service using MSI.

i.MX6 Root Ports implement the MSI Capability and may use MSI to request
service for events like PME, hotplug, AER, etc.  In addition, on i.MX6, the
MSI Enable bit controls delivery of MSI interrupts from components below
the Root Port.

Prior to f3fdfc4a ("PCI: Remove host driver Kconfig selection of
CONFIG_PCIEPORTBUS"), enabling CONFIG_PCI_IMX6 automatically also enabled
CONFIG_PCIEPORTBUS, and when portdrv claimed the Root Ports, it set the MSI
Enable bit so it could use PME, hotplug, AER, etc.  As a side effect, that
also enabled delivery of MSI interrupts from downstream components.

The imx6q-pcie driver itself does not depend on portdrv, so set MSI Enable
in imx6q-pcie so MSI from downstream components works even if nobody uses
MSI for the Root Port events.

Fixes: f3fdfc4a ("PCI: Remove host driver Kconfig selection of CONFIG_PCIEPORTBUS")
Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Sven Van Asbroeck <TheSven73@googlemail.com>
Tested-by: Trent Piepho <tpiepho@impinj.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>