dt-bindings: net: aspeed: add AST2700 MDIO compatible

dt-bindings: net: aspeed: add AST2700 MDIO compatible

Add "aspeed,ast2700-mdio" compatible to the binding schema with a fallback
to "aspeed,ast2600-mdio".

Although the MDIO controller on AST2700 is functionally the same as the
one on AST2600, it's good practice to add a SoC-specific compatible for
new silicon. This allows future driver updates to handle any 2700-specific
integration issues without requiring devicetree changes or complex
runtime detection logic.

For now, the driver continues to bind via the existing
"aspeed,ast2600-mdio" compatible, so no driver changes are needed.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Jacky Chou <jacky_chou@aspeedtech.com>
Link: https://patch.msgid.link/20251120-aspeed_mdio_ast2700-v2-1-0d722bfb2c54@aspeedtech.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>