drivers/net/phy: add driver for Microchip LAN867x 10BASE-T1S PHY

This patch adds support for the Microchip LAN867x 10BASE-T1S family
(LAN8670/1/2). The driver supports P2MP with PLCA.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Ramón Nordin Rodriguez <ramon.nordin.rodriguez@ferroamp.se>
Signed-off-by: David S. Miller <davem@davemloft.net>