dt-bindings: imx6q-pcie: Add "vph-supply" for PHY supply voltage

dt-bindings: imx6q-pcie: Add "vph-supply" for PHY supply voltage

The i.MX8MQ PCIe PHY can use either a 1.8V or a 3.3V power supply.  Add a
"vph-supply" property to indicate which regulator supplies power for the
PHY.

Link: https://lore.kernel.org/r/1622771269-13844-2-git-send-email-hongxing.zhu@nxp.com
Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>