net: dsa: microchip: lan937x: register mdio-bus

This patch register mdio-bus for the lan937x series switch. mdio read
and write uses the vphy for accessing the phy register.

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