net: phy: aquantia: move to separate directory

Move aquantia PHY driver to separate driectory in preparation for
firmware loading support to keep things tidy.

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