net: dsa: mt7530: fix module autoloading for OF platform drivers

It's required to create a modules.alias via MODULE_DEVICE_TABLE helper
for the OF platform driver. Otherwise, module autoloading cannot work.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>