net: phy: move at803x PHY driver to dedicated directory

In preparation for addition of other Qcom PHY and to tidy things up,
move the at803x PHY driver to dedicated directory.

The same order in the Kconfig selection is saved.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20240129141600.2592-2-ansuelsmth@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>