net: dsa: microchip: add support for phylink mac config

This patch add support for phylink mac config for ksz series of
switches. All the files ksz8795, ksz9477 and lan937x uses the ksz common
xmii function. Instead of calling from the individual files, it is moved
to the ksz common phylink mac config function.

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