Clear CLKREQ# override breaks functionality

[Franz Schnyder]

Hi Richard,

While integrating the `supports-clkreq` DT property on our iMX95-based
SoM, we had failures in our CI on one of the two M.2 PCIe slots on our
development board. 
The failing slot uses a card that does not advertise L1 PM substates.
This issue comes from commit a152a90f5390 ("PCI: imx6: Clear CLKREQ# 
override if 'supports-clkreq' DT property is available"), which clears
the CLKREQ# override based only on the DT property.

It seems that clearing the CLKREQ# override should happen only
when the driver knows that the downstream device advertises L1 PM 
Substates. Otherwise CLKREQ# should remain asserted to keep compatibility
with cards that do not support L1 PM Substates.

Thoughts?

Kind Regards

Franz