net: phy: microchip_t1: add cable test support for lan87xx phy

Add a basic cable test (diagnostic) support for lan87xx phy.
Tested with LAN8770 for connected/open/short wires using ethtool.

Signed-off-by: Yuiko Oshino <yuiko.oshino@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>