drivers/net/phy: add driver for the onsemi NCN26000 10BASE-T1S PHY

This patch adds support for the onsemi NCN26000 10BASE-T1S industrial
Ethernet PHY. The driver supports Point-to-Multipoint operation without
auto-negotiation and with link control handling. The PHY also features
PLCA for improving performance in P2MP mode.

Signed-off-by: Piergiorgio Beruto <piergiorgio.beruto@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>