PCI: imx6: Assert i.MX8MM CLKREQ# even if no device present

PCI: imx6: Assert i.MX8MM CLKREQ# even if no device present

The CLKREQ# signal is an open drain, active low signal that is driven
low by the remote Endpoint device. But it might not be driven low if no
Endpoint device is connected.

On i.MX8MM PCIe, phy_init() may fail and system boot may hang if no
Endpoint is connected to assert CLKREQ#.

Handle this as on i.MX8MQ, where we explicitly assert CLKREQ# so the
PHY can be initialized.

Link: https://lore.kernel.org/r/1645672013-8949-1-git-send-email-hongxing.zhu@nxp.com
Fixes: 178e244cb6e2 ("PCI: imx: Add the imx8mm pcie support")
Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>