net: phy: Add 10-BaseT1L registers

The 802.3gc specification defines the 10-BaseT1L link
mode for ethernet trafic on twisted wire pair.

PMA status register can be used to detect if the phy supports
2.4 V TX level and PCS control register can be used to
enable/disable PCS level loopback.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Alexandru Tachici <alexandru.tachici@analog.com>
Signed-off-by: David S. Miller <davem@davemloft.net>