net: phy: move PHY package code from phy_device.c to own source file

This patch is the first step in moving the PHY package related code
to its own source file. No functional change intended.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://patch.msgid.link/57df5c19-fbcd-45a7-9afd-cd4f74d7fa76@gmail.com


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