PCI: imx6: Add CLKREQ# override to enable REFCLK for i.MX95 PCIe

PCI: imx6: Add CLKREQ# override to enable REFCLK for i.MX95 PCIe

The CLKREQ# is an open drain, active low signal that is driven low by
the card to request reference clock. It's an optional signal added in
PCIe CEM r4.0, sec 2. Thus, this signal wouldn't be driven low if it's
not exposed on the slot.

On the i.MX95 EVK board, REFCLK to the host and endpoint is gated by this
CLKREQ# signal. So if the CLKREQ# signal is not driven by the endpoint, it
will gate the REFCLK to host too, leading to operational failure.

Hence, enable the REFCLK on this SoC by enabling the CLKREQ# override using
imx95_pcie_clkreq_override() helper during probe. This override should only
be cleared when the CLKREQ# signal is exposed on the slot.

Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
[mani: reworded description]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Tested-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20251015030428.2980427-11-hongxing.zhu@nxp.com