net: phy: at803x: move specific DT option for at8031 to specific probe

Move specific DT options for at8031 to specific probe to tidy things up
and make at803x_parse_dt more generic.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>