dt-bindings: net: dwmac: fix 'mac-mode' type

The 'mac-mode' property is similar to 'phy-mode' and 'phy-connection-type',
which are enums of mode strings.

The 'dwmac' driver supports almost all modes declared in the 'phy-mode'
enum (except for 1 or 2). But in general, there may be a case where
'mac-mode' becomes more generic and is moved as part of phylib or netdev.

In any case, the 'mac-mode' field should be made an enum, and it also makes
sense to just reference the 'phy-connection-type' from
'ethernet-controller.yaml'. That will also make it more future-proof for new
modes.

Fixes: 9c15d3597c62 ("dt-bindings: net: dwmac: document 'mac-mode' property")
Signed-off-by: Alexandru Ardelean <[e-mail]>
Reviewed-by: Andrew Lunn <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/devicetree/bindings/net/snps,dwmac.yaml

···
     const: stmmaceth
 
   mac-mode:
-    maxItems: 1
+    $ref: ethernet-controller.yaml#/properties/phy-connection-type
     description:
       The property is identical to 'phy-mode', and assumes that there is mode
       converter in-between the MAC & PHY (e.g. GMII-to-RGMII). This converter