PCI: imx6: Clear CLKREQ# override if 'supports-clkreq' DT property is available

PCI: imx6: Clear CLKREQ# override if 'supports-clkreq' DT property is available

CLKREQ# is an optional reference clock request signal defined by the PCIe
CEM and M.2 specifications to request REFCLK and exit the L1 Substates. The
imx6 controller driver so far forced the CLKREQ# signal to low by enabling
the CLKREQ# override logic as the slots do not expose this signal.

Now, there are board designs coming up exposing this signal to the endpoint
devices. This is identified using the 'supports-clkreq' DT property in the
controller node.

So when the DT node has this property, clear the CLKREQ# override after
link up in host_post_init() callback to allow the endpoint to drive the
CLKREQ# signal.

Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
[mani: squashed the imx8mm_pcie_clkreq_override helper patch & reworded description]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>