net: phy: at803x: make at8031 related DT functions name more specific

Rename at8031 related DT function name to a more specific name
referencing they are only related to at8031 and not to the generic
at803x PHY family.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>