The GEM IPs within Microchip p64h have their MDIO controllers
unconnected from any physical pin.

When compiling a p64h device tree with a phy on a GEM node with
CHECK_DTBS=1, this generates an error like:

```
linux/arch/riscv/boot/dts/microchip/p64h-hb130x.dtb:
ethernet@40004180000 (microchip,p64h-gem): ethernet-phy@0: False
schema does not allow {'reg': [[0]]}
	from schema $id:
http://devicetree.org/schemas/net/cdns,macb.yaml#
```

Signed-off-by: Charles Perry <charles.perry@microchip.com>
---
 Documentation/devicetree/bindings/net/cdns,macb.yaml | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/Documentation/devicetree/bindings/net/cdns,macb.yaml b/Documentation/devicetree/bindings/net/cdns,macb.yaml
index dff350302098..be66cc9a42fd 100644
--- a/Documentation/devicetree/bindings/net/cdns,macb.yaml
+++ b/Documentation/devicetree/bindings/net/cdns,macb.yaml
@@ -197,6 +197,17 @@ allOf:
       required:
         - phys
 
+  - if:
+      properties:
+        compatible:
+          contains:
+            const: microchip,p64h-gem
+    then:
+      patternProperties:
+        "^ethernet-phy@[0-9a-f]$": false
+      properties:
+        mdio: false
+
 unevaluatedProperties: false
 
 examples:
-- 
2.47.3