net: mvneta: Add support for 98DX2530 Ethernet port

The 98DX2530 SoC is similar to the Armada 3700 except it needs a
different MBUS window configuration. Add a new compatible string to
identify this device and the required MBUS window configuration.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>