dt-bindings: phy: phy-imx8-pcie: Add binding for the pad modes of imx8 pcie phy

Add binding for reference clock PAD modes of the i.MX8 PCIe PHY.

Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Tim Harvey <tharvey@gateworks.com>
Tested-by: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1638432158-4119-2-git-send-email-hongxing.zhu@nxp.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>