dt-bindings: net: cdns,macb: Add external REFCLK property

REFCLK can be provided by an external source so this should be exposed
by a DT property. The REFCLK is used for RMII and in some SoCs that use
this driver the RGMII 125MHz clk can also be provided by an external
source.

Signed-off-by: Ryan Wanner <[email protected]>
Acked-by: Conor Dooley <[email protected]>
Link: https://patch.msgid.link/d558467c4d5b27fb3135ffdead800b14cd9c6c0a.1752510727.git.Ryan.Wanner@microchip.com
Signed-off-by: Jakub Kicinski <[email protected]>

Author: ryan-wanner

Documentation/devicetree/bindings/net/cdns,macb.yaml

@@ -115,6 +115,13 @@ properties:
   power-domains:
     maxItems: 1
 
+  cdns,refclk-ext:
+    type: boolean
+    description:
+      This selects if the REFCLK for RMII is provided by an external source.
+      For RGMII mode this selects if the 125MHz REF clock is provided by an external
+      source.
+
   cdns,rx-watermark:
     $ref: /schemas/types.yaml#/definitions/uint32
     description: