dt-bindings: PCI: dwc: Add external reference clock input

Add external reference clock input "extref" for a reference clock that
comes from external crystal oscillator.

Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20251211064821.2707001-2-hongxing.zhu@nxp.com

Author: Richard Zhu

Documentation/devicetree/bindings/pci/snps,dw-pcie-common.yaml

@@ -105,6 +105,12 @@ properties:
             define it with this name (for instance pipe, core and aux can
             be connected to a single source of the periodic signal).
           const: ref
+        - description:
+            Some dwc wrappers (like i.MX95 PCIes) have two reference clock
+            inputs, one from an internal PLL, the other from an off-chip crystal
+            oscillator. If present, 'extref' refers to a reference clock from
+            an external oscillator.
+          const: extref
         - description:
             Clock for the PHY registers interface. Originally this is
             a PHY-viewport-based interface, but some platform may have