net: phy: Move of_mdio from drivers/of to drivers/net/mdio

Better place for of_mdio.c is drivers/net/mdio.
Move of_mdio.c from drivers/of to drivers/net/mdio

Signed-off-by: Calvin Johnson <calvin.johnson@oss.nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>