net: dsa: microchip: lan937x: add phylink_mac_config support

This patch add support for phylink_mac_config dsa hook. It configures
the mac for MII/RMII modes. The RGMII mode will be added in the future
patches.

Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>