net: dsa: microchip: lan937x: add phylink_get_caps support

The internal phy of the LAN937x are capable of 100Mbps Full duplex. The
xMII port of switch is capable of 10Mbps Full & Half Duplex, 100Mbps
Full & Half Duplex and 1000Mbps Half duplex. xMII port also supports Tx
and Rx Flow control.

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