PCI: imx6: Save and restore root port MSI control in suspend and resume

PCI: imx6: Save and restore root port MSI control in suspend and resume

The imx6 PCI host controller suffers from a HW integration bug whereby
the MSI enable bit in the root port MSI capability enables/disables MSIs
interrupts for all downstream components in the PCI tree.

This requires, as implemented in

75cb8d20c112 ("PCI: imx: Enable MSI from downstream components")

that the root port MSI enable bit should be set in order for downstream
PCI devices MSIs to function.

The MSI enable bit programming might be lost during the suspend and
should be re-stored during resume.

Save the MSI control during suspend and restore it in resume.

Link: https://lore.kernel.org/r/1670479534-22154-1-git-send-email-hongxing.zhu@nxp.com
Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
[lpieralisi@kernel.org: commit log]
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>