net: dsa: microchip: lan9371/2: add 100BaseTX PHY support

On the LAN9371 and LAN9372, the 4th internal PHY is a 100BaseTX PHY
instead of a 100BaseT1 PHY. The 100BaseTX PHYs have a different base
register offset.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>