realtek: setup cpu port in soc_info structure

realtek: setup cpu port in soc_info structure

A lot of soc_info usage has been reorganized. Nevertheless there
are some consumers left. A very critical one is the dsa/qos coding.
It makes use of the cpu_port in this shared structure. This is
totally broken as that info is never properly initialized. Fill
the cpu_port according to the identified hardware.

Remark: Looking at the prom.c history soc_info.cpu_port was never
setup since the beginning of time. So no "fixes" tag here.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/21327
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>