According to the PCI-E standard the PERST# signal (reset-gpio in fsl,imx6* compatible dts) should be kept asserted for at least 100 usec before the PCI-E refclock is stable, should be kept asserted for at least 100ms after the power rails are stable and the host should wait at least 100 msec after it is de-asserted before accessing the configuration space of any attached device.
