i.MX8MM PCIe has the PHY. Add a PHY phandle and name properties
in the binding document.

Link: https://lore.kernel.org/r/1638432158-4119-4-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>
Reviewed-by: Rob Herring <robh@kernel.org>