Commit f4fb63d2ab ("ipq40xx: 5.10: move AR40xx to MDIO drivers") moved
the ar40xx driver files to kernel version specific directories to place
them in different subdirectory in kernel tree. But now kernel 5.4 is
gone and there is no reason to keep them separate. Move them back to
common files/ directory.
Signed-off-by: Sungbo Eo <mans0n@gorani.run>
