PCI: imx6: Keep Root Port MSI capability with iMSI-RX to work around hardware bug

On NXP i.MX7D, i.MX8MM, and i.MX8MQ chipsets, MSIs from the endpoints won't
be received by the iMSI-RX MSI controller if the Root Port MSI capability
is disabled.

Even though the Root Port MSIs won't be received by the iMSI-RX controller
due to design, these chipsets have some weird hardware bug that prevents
the endpoint MSIs from reaching when the Root Port MSI capability is
disabled.

Hence, introduce a new flag, 'dw_pcie_rp::keep_rp_msi_en', set it for the
above mentioned SoCs, and always keep the Root Port MSI capability when
this flag is set.

Note that by keeping Root Port MSI capability, Root Port MSIs such as AER,
PME and others won't be received by default. So users need to use
workarounds such as passing 'pcie_pme=nomsi' cmdline param.

Fixes: f5cd8a929c825 ("PCI: dwc: Remove MSI/MSIX capability for Root Port if iMSI-RX is used as MSI controller")
Suggested-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
[mani: commit log]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
[bhelgaas: fix typos]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20260331085252.1243108-1-hongxing.zhu@nxp.com

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