net: dsa: microchip: lan937x: add phylink_mac_link_up support

This patch add support for phylink_mac_link_up. It configures the mac
for the speed, flow control and duplex mode.

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