net: phy: move realtek PHY driver to its own subdirectory

In preparation of adding a source file with hwmon support, move the
Realtek PHY driver to its own subdirectory and rename realtek.c to
realtek_main.c.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/c566551b-c915-4e34-9b33-129a6ddd6e4c@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>