realtek: phy: rename and relocate module

realtek: phy: rename and relocate module

The downstream Realtek phy module is currently known as rtl83xx-phy.c
and its kernel config REALTEK_SOC_PHY. It has been simplified, cleaned
and now aligns to Realtek main module (upstream Realtek phy). It is no
longer tied to the Realtek switch SoC but serves as generic module for
1Gbit multiport phys. Adapt it as follows:

- place it into the realtek folder aside its upstream sibling
- rename it to realtek_multiport.c
- remove SoC dependency in Kconfig and Makefile
- change kernel configs for the targets accordingly

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/21929
Signed-off-by: Robert Marko <robimarko@gmail.com>