i.MX8MM PCIe works mostly like the i.MX8MQ one, but has a different PHY
and allows to output the internal PHY reference clock via the refclk pad.
Add the i.MX8MM PCIe support based on the standalone PHY driver.

Link: https://lore.kernel.org/r/1640312885-31142-2-git-send-email-hongxing.zhu@nxp.comTested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Tested-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Tim Harvey <tharvey@gateworks.com>