drivers/net/phy: add helpers to get/set PLCA configuration

This patch adds support in phylib to read/write PLCA configuration for
Ethernet PHYs that support the OPEN Alliance "10BASE-T1S PLCA
Management Registers" specifications. These can be found at
https://www.opensig.org/about/specifications/

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>