dt-bindings: imx6q-pcie: Add PHY phandles and name properties

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

Link: https://lore.kernel.org/r/[e-mail]
Tested-by: Marcel Ziswiler <[e-mail]>
Tested-by: Tim Harvey <[e-mail]>
Signed-off-by: Richard Zhu <[e-mail]>
Signed-off-by: Lorenzo Pieralisi <[e-mail]>
Reviewed-by: Tim Harvey <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/pci/fsl,imx6q-pcie.yaml

···
     enum: [1, 2, 3, 4]
     default: 1
 
+  phys:
+    maxItems: 1
+
+  phy-names:
+    const: pcie-phy
+
   reset-gpio:
     description: Should specify the GPIO for controlling the PCI bus device
       reset signal. It's not polarity aware and defaults to active-low reset