net: phy: add qca8081 ethernet phy driver

qca8081 is a single port ethernet phy chip that supports
10/100/1000/2500 Mbps mode.

Add the basic phy driver features, and reuse the at803x
phy driver functions.

Signed-off-by: Luo Jie <luoj@codeaurora.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>